Please pardon the crazy seamstress lecture, but make sure your pants fit first!
I'm not sure if there are particular commercial brands designed to leave room for more curve in the butt, but there's no sense trying to change the shape of your body if the problem is actually with your clothes.
I thought for a very long time that despite being a pear shape, I had a flat butt: wide, but without depth. (When people talk about butt size I'm never entirely sure whether they mean butt size or hip width. Usually it's hip width that people dislike, rather than what would accurately be
butt size.) Once I started sewing and altering patterns to actually fit, I realized that it wasn't my butt that was flat, it was just my
pants that had been flat. It uses less yardage to have straighter pattern pieces (which is why clothing manufacturers do it,) but at the expense of room for a butt. Wearing properly cut pants, I've got more of a butt, but actually look thinner because when badly designed patterns flatten your butt, the mass has to go somewhere, so it tends to go out to the sides.