The ones I've always seen at good gyms and PT clinics are Perform Better brand. Their main site appears to be down right now (it was fine a week ago when I bought one), but they also sell on Amazon.

https://www.amazon.com/Perform-Better-Exercise-Superband-1-5/dp/B018M58DC6

Main site in case it goes back up: https://www.performbetter.com/