What is a good brace that prevents shoulder dislocations?

Not great options: Most people who want a brace for shoulder instability use it for sports. Those braces restrict shoulder motion to a safe zone where the shoulder won't dislocate. They can be very restrictive. Depending on the sport and position played, this restriction may or may not allow appropriate participation. If you need a brace for everyday activities, see your ortho doc.

Stabilizing brace.: There are a number of "shoulder stabilizing braces" that can be found on the internet. It should be advertised as appropriate for anterior shoulder dislocations.