Two years ago, after 40 years at Richmond and Montrose, Ben Russell and Vikki Trammell moved Art Supply to a two-story commercial space in Midtown. Respected as nurturers of the local art scene as well as the preferred source for top-grade fine-arts supplies, the couple remodeled the upstairs of 2711 Main into 22 studios and seven loft apartments for artists, writers and musicians. As it happened, all but three of the units are leased to painters and sculptors, who have an artist-friendly place to live and work, as well as the opportunity to show and sell their creations at open houses sponsored by Russell and Trammell twice a year. (The next one is scheduled for November 25 and 26.) Meanwhile, downstairs, in a large gritty, no-frills room that smells of sawdust, Art Supply continues to buck the trend in art supply stores by catering its inventory to the serious artist only. Unlike the chain stores, Art Supply doesn't stock arts-and-crafts materials. Nor does it employ minimum-wage slaves who know nothing about art or art supplies. Art Supply is staffed by working artists who know what they're selling because, more than likely, they've used it themselves.