There are lots and lots of jazz-related t-shirts on eBay, mostly the icons. I don't have the desire to buy a Bill Evans shirt to make everyone around me know that I'm a fan.
I see t-shirts sold at gigs sometimes, but it's usually not something I'd wear, although I did buy the Mingus Big Band shirt last year(or was it the year prior) at the centennial. Many, if not most jazz clubs and record stores sell t-shirts with their imagery. I have several.
I give my old, worn and holed music-related t-shirts to my daughter. She has no interest in the musicians and bands pictured on the shirts (like Floyd, Stones, some alternative stuff from way back), but likes to wear vintage clothing, and regularly shops at Savers, along with her girlfriends. Sometimes she brings home cool things from a long time ago, when I was young.