Normally you buy the suit in the appropriate jacket size and the appropriate trousers come with it. You can see the measurements for the trousers for each size in the Size Advisor pane. Mixing-and-matching different jacket/trouser sizes isn't very common in suit buying although I don't know how accomodating SuitSupply are for it.
Really? That's weird because I'm a 38s suit jacket but a 31 in pants. The 38s Washington jacket comes with a 35 pants...