Firstly, always buy a suit with well fitting pants. Holding up suit pants, or any pants for that matter, with a belt is a bad look. If there happens to be a great deal on a suit in a store with a slightly larger waist than that perfect fit, either A- get out of that shop and save your money, or B- find and build a strong relationship with a good tailor. If you already own a number of suits that are too big on the waist and require a belt, get your tailor to do his best job of taking them in. In the case that you have recently lost a large amount of weight and your suit trousers look more like a sleeping bag then give them to your local Salvation Army and go buy new ones. There is only so much a good tailor can do.