I've got some Frugi (cut4cloth) trousers which are great but very expensive. It's worth signing up for emails as they sometimes have up to 70% off in sales. We've been given some Sainsbury's jeans which seem OK but I rely a lot on jogging style trousers. Dungarees are also more roomy around the bum but don't work well for us as ds is long in the body so either the legs are way too long or the crotch poppers pop open
I hope you find something that works.