On a plum sofa in a cedar-sided house sits Matthew Lesko, a wiry, alert man with the key to a treasure chest. Mr. Lesko is onto something. Something large. He stumbled across it in 1975 when a friend asked him to find out why the price of Maine potatoes was then double the norm. After a few phone calls to the Department of Agriculture, Mr. Lesko found his key - a bureaucrat who had ''spent his whole working life studying the very thing I was looking for.''
The problem, he found, was not in discovering such information. The problem was in getting too much. He called his friend back - and walked away with a fat consultant's fee.
Since then, Mr. Lesko has started a consulting firm called Washington Researchers. Its sole purpose is to find informed bureaucrats and connect their knowledge with business people who need it. He has also written a $20 paperback, ''Information USA'' (Viking Press), telling how he does it - a book that has made the New York Times best-seller list.
''The International Trade Commission does 300 market studies each year, because they're worried about the effect of imports on products in the US,'' he says. ''The fallout of those studies is that businesses can let the government do their market research - if they only knew it was there.''
Mr. Lesko has also figured out how to use the government to help finance a new business (''the Catalog of Federal Domestic Assistance has a shopping list for all monies''), how to run a day-care center (''look to the Early Childhood Clearinghouse''), and how to convert a business to a franchise (''I found a lawyer at the Federal Trade Commission who knew all about it - why pay some other lawyer $20 an hour for what you can get for free?''). He knows how to get free firewood, free Christmas trees for a nonprofit group, free manure for his garden, and free fish for his pond, all from the US government.
All this information - and much, much more - is like ''oil in the ground,'' he believes. ''The government spends very little money on advertising, so people just don't know what's available. I shouldn't be in business - the government should be telling people these things.''
He also thinks that if people had more access to the sorts of helpful information produced by the US government, they would ''feel better'' about Washington. The Leskos - Matthew, his wife, Wendy, and toddler Morgan - took ''a van full of government freebies around the country this summer and found that most people really appreciated the information they got,'' he says. Ninety-four percent of those he surveyed, however, felt that the government does not adequately tell them about programs that could help them.
''I figure most people feel like I always did,'' he says. As a young man in his hometown of Wilkes-Barre, Pa., he worked for his father's garment factory. ''I thought the government was something special, for either the very, very rich or the very, very poor. I couldn't believe when I got here that I could pick up the phone and find out about a bill on Capitol Hill by dialing the same number that some $400,000 lawyer uses.''
He's also been pleasantly surprised by the helpfulness of the average government bureaucrat. ''These people will talk to me - a nobody - for hours about their subject, and then take me around to someone else in the department who has the statistics,'' he reports.
Part of his success in uncovering this treasure, he feels, comes from ''friendliness. A bureaucrat gets the same salary whether he's helpful to you or not. So you have to be humble, be grateful, treat them like the experts they are.'' He advises thank-you notes and passing back any extra information you might receive that could help your helper.
He also advises using the phone. ''There are no smokestacks downtown,'' he says, pointing toward Washington, D.C. ''All they produce is paper. So if you write a letter, you're just adding to the heap. But a friendly phone call from one person to another will stand a better chance of getting you what you need.''
Those who live outside the capital area can use ''local offices of the government departments or their congressmen's office - that's a great resource - or a few 800 numbers that have been set up.''
Numbers like these:
College money: 800-638-6700 (Maryland: 800-492-6602). Department of Education.
Buying an auto: 800-424-9393 (District of Columbia: 202-426-0123). National Highway Traffic Safety Administration, facts on new or used cars.
Starting a business: 800-368-5855 (District of Columbia: 202-653-7561). Small Business Administration's Answer Desk.
Banking questions: 800-424-5488 (District of Columbia: 202-389-4221). Federal Deposit Insurance Corporation, for problems with credit or other bank services.
Mr. Lesko has also uncovered innumerable government freebies such as these:
Golden-age passport: Anyone over 62 can get this lifetime entrance permit to federal parks, monuments, and recreation areas through the National Park Service's regional offices or at the Information Office, Department of the Interior, 18th and C Streets NW, Washington, D.C. 20240.
Camping guide: The country's 154 national forests and 19 national grasslands are described in a brochure from the Office of Information, Forest Service, Department of Agriculture, Room 3238, South Building, Washington, D.C. 20250.
Free toys: Buttons, decals, posters, and children's educational materials, all related to ecology, available from the Office of Public Awareness, Environmental Protection Agency, 401 M Street SW, Washington, D.C. 20460.
Government surplus property: Items are discounted from their original value; some are donated to nonprofit organizations. Contact regional surplus sales offices as follows: (1) General Services Administration, Surplus Sales Branch, 7 th and D Streets NW, Washington, D.C. 20407; (2) General Services Administration , Surplus Sales Branch, 75 Spring Street, Atlanta, Ga. 30303; (3) General Services Administration, Surplus Sales Branch, 525 Market Street, San Francisco, Calif. 94105. Some surplus is also sold by auction through the Department of Defense, Surplus Sales, PO Box 1370, Battle Creek, Mich. 49016.
Mr. Lesko also recommends the following as the starting point for research projects:
* Program Information Center, US Department of Housing and Urban Development, 451 7th Street SW, Washington, D.C. 20410. (202) 755-6420. The center will identify programs on all aspects of housing - building, buying, selling, adding, and so on.
* Bureau of Industrial Economics, US Department of Commerce, Room 4878, Washington, D.C. 20230. (202) 377-4356. Over 100 industry analysts can supply information, market studies, and forecasts about specific companies and industries.
* National Referral Center, Library of Congress, Washington, D.C. 20540. (202 ) 287-5670. The staff will find an expert or organization for you on any subject.
* Bill Status Office, US Capitol, House Annex No. 2, Room 2650, Washington, D.C. 20515. (202) 225-1772. The office can determine if there is any current legislation on specific subjects.