The Incrediwear braces already mentioned are good and have held up very well, for years.
https://www.amazon.com/gp/product/B0...TF8&th=1&psc=1

I also have a pair of these and they're at least as good, maybe a little more comfortable.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

These both do have a very light and flexible side brace running down each side, but those are there to keep them from bunching up - they don't provide much in the way of actual support. Both of these work well, and I'm at the stage where I need them all the time while cycling.