I have this exact same problem. It makes buying pants so difficult and frustrating. They will fit fine everywhere but I'll have so much space in the back area of the waist. For jeans, I'd suggest AG or Blue Cult jeans. They are on the expensive side but they fit so well that it's worth it. Also, shop around. I've found both of these types of jeans at Annie Sez for a fraction of the regular retail price. Express jeans also fit me well and they come in S, M & L, which is always great because I hate getting jeans hemmed.
I personally don't nlike Gap jeans. I think they're too stiff and rough.