Nodaisho, the thing is, you can't rely on picking up something from a shelf or a rack and expecting it to just fit you the way you want. 9 times out of 10 it won't work that way, and there will be several things about each individual suit or really anything (dress shirts, trousers) that you are unhappy with, casual clothes excluded. All of the problems that thousands of men have had with their suits have been solved over the years by tailoring. In the states, a lot of dry cleaners will also do alterations, but you could ask at the place you buy the suit for recommendations. If you fork over a few hundred bucks for your suit, chances are they will offer tailoring as part of the sale or like a $20-40 add-on in the store. Even tux rental places do fittings and alterations for dudes wearing them for 2 hours for their weddings. It is a pretty standard thing, I think.
And while I am making a lot of assumptions about menswear here, I know they do this for women. Any place (like David's Bridal) worth its salt that sells prom dresses (and probably also wedding dresses) offers alterations either included in the price of the garment or for a small extra fee. Since suits are just as finicky and just as important as the dresses they are supposed to stand next to, and a lot more subtle in what determines their quality, it makes sense that customising is the way of the game.
Basically what I am saying is, if you have a slim waist, tailoring will probably give you that silhouette regardless.