Johnny Manziel's wigged-out Las Vegas adventure is not just urban legend -- it's now a major windfall for the makers of some clever new Manziel gear. 

Two different Ohio based t-shirt companies tell TMZ Sports they're cashing in on the reports Johnny Football hit the strip last weekend wearing a blonde wig and mustache.  The Internet's taken to calling him "Billy Vegas" or "Billy Football." 

Tony Madalone, the owner of Fresh Brewed Tees, says they started cranking out tees with blonde "Billy" in a Browns jersey the day after the story came out.  He says they sold 3,000 of 'em in 36 hours ... one of their bestsellers ever. 

Meanwhile, Lamp Apparel's "Billy Football" tees -- which play off the famous Unabomber sketch -- are also flying off shelves, according to their rep, Brandon Fuss-Cheatham.