Where can i find trousers that fit?!!!?

I am between a size 10 - 12 and i have quite a big backside!!! so i find it hard to find trousers that stay on my hips, and dont start falling down when i sit or bend over!!! it's like i need a size 10 - 12 hip, and a size 16 bum fitting!!! AGHHHH!!!! (i have joined the gym to tone up by the way!!)