If you're looking for the Old West, here's one place you can find it

When you stand on the hard-packed-dirt main street of Old Tucson, you feel as if you've stepped right into an urban cowboy's fantasy. In front of you is a gun and fist-fight: rugged, leather-chaps-clad good guys taking on some bank-robbing desperadoes, re-creating what Hollywood has taught the world to accept as standard Western frontier life.

If it seems like a scene straight out of the movies, that's no coincidence. Old Tucson was built in 1939 for the Columbia Pictures epic ''Arizona.'' Since then, so many westerns have been filmed there it has come to be called ''Hollywood in the Desert.'' Old Tucson is the setting of such other movies as ''Rio Bravo'' and ''Posse,'' as well as television shows with a frontier look: ''Little House on the Prairie,'' ''Chaparral Family,'' ''More Wild Wild West.'' Often you can watch a scene being filmed as you tour its weathered wood-frame and adobe buildings. Even if there's no movie crew on hand, there are the rough-and-tumble western shows to put you in the spirit of Tucson at another time.

Old Tucson is a family fun park (open daily 9:30 a.m. to 5 p.m.) ''100 years and 12 miles from the city of Tucson,'' according to its publicity brochure. And Tucson is a modern city that, even if Old Tucson weren't its neighbor, would wear an overlay of the Old West. After all, it saw its last Apache uprising as recently as 1911.

Lately it's seeing another kind of uprising - this one by tourists. They come here from everywhere, looking for . . . well, ''I'm looking for the West,'' said Gunther, a visitor from West Germany - ''for cowboys.''

Gunther could not have looked more like a cowboy if he had been born in Arizona. He was leaning against the corral at Tanque Verde. With that backdrop, his Levi's and wide-brimmed hat, his leather vest and pointed-toe, high-heeled boots, fitted right in.

Most of us gathered around Tanque Verde's corral were dressed pretty much as Gunther was. Most of us were at Tanque Verde for the same reason, too. We were here not only to see cowboys but to be cowboys (and cowgirls).

Tanque Verde (Route 8, PO Box 66, Tucson, Ariz. 85710) is a ''guest ranch.''

It and other working ranches here started taking paying guests back in the 1920s, when Easterners, visiting Arizona for its more temperate winters, thought it would be fun to try working with cattle.

Soon more and more ranches began to take in paying guests - dudes, they were called then - and, for a while, dude ranches flourished like calves in spring. Then Tucson's population exploded (as did the population in much of the West), and the wide-open spaces became housing developments. Today there are only a couple of dozen guest ranches in this area.

Bob Cote, owner-manager of Tanque Verde, calls them ''living museums of our country's past.''

Not that these places don't offer modern conveniences, even comforts. At Tanque Verde the original ranch house serves as headquarters. That's where you check in. There's a cozy living room with fireplace and a large dining room that serves the sizable meals you're going to want after a day in the saddle. Even better than their size, though, is their appeal: The kitchen is excellent. Especially good, of course, is the beef you'd expect to find in this part of the country, once overrun with steaks on the hoof.

Scattered over the hilly grounds are clusters of cabins, most of them suites, many with their own fireplaces. They are private; some even give a feeling of seclusion; but all are within easy walking distance of the main house and the corral.

Although you'll find a swimming pool and tennis courts on the grounds, it's the corral that gets most visitors' attention. Tanque Verde boards some 100 horses, and you can be in the saddle pretty much all day if you want to be.

It was at 6:30 a.m. that I met Gunther. We and about 50 others were watching the wranglers get horses ready for Tanque Verde's weekly breakfast ride.

It is perhaps in the early hours of the day that the Old West can most easily take precedence over the modern one in the mists of our minds.

The horses are snorting, pawing the ground, prancing a few steps with the buildup of energy over the long night. The leather creaks as we lift ourselves into the saddle.

The waxing sun hasn't had time to gather any heat. It will be a while before it does, because gray veils of cloud still hold more power in a frowning sky.

The horses file out of the corral, and we move onto the trail, which stretches past a ragged line of giant saguaro cactuses. The trail is sandy, but not the soft, drifting stuff you usually think of when you think desert. The hot summer has baked this hard. Even today, with fall coming fast and bringing with it unseasonable rain, it's dry-looking and cracked.

We move along, rocking slightly, looking for jackrabbits, javelinas (wild pigs), and snakes, and seeing the same barren countryside the frontiersmen must have seen.

Finally day envelops us, but civilization doesn't. It takes an hour of riding to see our first building, a small square of fieldstone, and another half-hour to climb the long incline to reach it.

The chuck wagon beat us there and the cooking crew has breakfast ready: eggs, flapjacks, biscuits, and bacon. We deserve this largess; it's been a hard day in the saddle, and the day has barely begun. Anyway, since it took us an hour and a half to get here, we have to garner some energy for the ride back.

It isn't until we are in the saddle again and heading down the other side of the hill that we realize the ranch is right below us. We're only 10 minutes from our cabins and a long way from the West as we've been dreaming it this morning.

Still, maybe it wasn't just a dream.

I look at Gunther as he swaggers away from the corral. His legs are slightly bowed from straddling a very wide horse. His Levi's, covered with Arizona dust, have lost their newness. And his smile is very real.

For information on Tanque Verde and other guest ranches in the area, on Old Tucson, and on Tucson itself, write: The Metropolitan Tucson Convention and Visitors Bureau, Box 3028, Tucson, Ariz. 85702.