An abiding affection for true work clothing and military wear stems from an appreciation of its basic, low-cost quality. John Weitz, the designer, collects hunter's socks and loose work dungarees. ''Real dungarees,'' he says, ''are not jeans. The trick is to find the original.'' Mr. Weitz buys his work wear in Sag Harbor, L.I., and at the 59th Street Army/Navy Store (221 East 59th Street), where his salesman ''knows he can't horse me around with labels,'' he says.

'We're Good Lookers'

At Kaufman Surplus Inc. (319 West 42d Street), one of Manhattan's surviving work-wear specialty stores, Jim Korn, the owner, discusses his stock with the pride of an environmentalist ticking off species he's saved from extinction. The Five Brothers brand flap-pocket chambray shirts that Kaufman's sells for $12.99 ''took us a long time to find,'' Mr. Korn says. ''They are a disappearing item.'' He points out more endangered breeds: railroad engineer's striped cotton shirts, selling for $18.99 to $24.99, and chinos made to military specifications. ''Nobody can find them,'' Mr. Korn says, pointing to a rack of new and used pants in khaki, black, gray and white that sell for $15 to $22.50. ''We're good lookers,'' he explains.

According to Eugene Mark, president of the GMP Sales Company, a garment wholesaler in Manhattan, the basic work-wear business ''has petered out.'' Several factors account for its decline. Manufacturers frequently cite competition from imports made in countries where labor costs are low. ''The business is going to countries where you can pay 50 cents a day,'' says Henry Gitenstein, senior vice president of the Riverside Shirt and Underwear Corporation, which has its offices in Manhattan. The cost of all-natural fabric has also risen tremendously. When Morty Appleton, president of Wolf Appleton Inc., another Manhattan wholesale jobber, entered the business in the mid-1940's, cotton chambray shirts wholesaled for $7.50 a dozen, he recalled. The same quantity costs $102 at wholesale today.

Consumers also hastened the move away from natural fibers. ''Working people today wear sports clothes'' on the job, Mr. Appleton says. ''The same factories that made chinos are now making slacks that sell for two to three times as much.'' Markups and profits are accordingly higher. Thus familiar work-wear labels like Levi's, Lee and Wrangler now appear on a wide variety of ''related but more sophisticated'' clothes, according to Meredith Sheperd, a vice president of the Washington Manufacturing Company in Nashville, which markets the Dee-Cee line. Oshkosh B'Gosh Inc., the Wisconsin-based work-wear manufacturer, has also had great success in selling children's clothing in department stores. ''A basic work-clothes manufacturer of the past would not be in business today,'' says Martin Fine, the fourth-generation owner of M. Fine & Sons in Louisville, Ky., which makes all-cotton chambray, chamois, flannel and Western shirts bearing the Five Brothers label. ''We make whatever sells.''

Manufacturers explain that work-wear purchasers are primarily the wives of working men, who these days often work themselves, but in any case prefer the convenience of easy-care, blended fabrics. ''Permanent-press poly-cottons look better,'' Mr. Mark contends. ''People have to wash those things,'' adds Ann Richardson, merchandise manager of the Williamson-Dickie Manufacturing Company, a major work-wear manufacturer in Fort Worth. ''Our customer is not your natural-fiber yuppie. We've had an all-cotton khaki work pant, and it just doesn't sell.'' Prisons are a big purchaser of Riverside Shirt's all-cotton products, but, Mr. Gitenstein says, ''you've got a captive labor force'' to iron them.