There's probably a good company in your area. On top of helping to support your local economy, you also don't have to worry about shipping :).

Another vote for find a local company and support them. I also suggest buying samples from a few different companies to see what you like. Our gym has used several over the years and some are better than others for certain things. For example, one of them had really big hand openings that would have been perfect for a fighter to walk out in because they would easily fit over gloves, but they were annoying in regular wear because they just flopped around. There's a good chance that one of the students at your gym does T shirts and promo items and would be happy to help.

“Durable” can mean a lot of things—are we talking Carhartt-level tough, or something more casual?

Something more casual I think. i don't believe we need anything to be made of Kevlar lol