What you're looking for are called "pressure tanks". They're used with well water systems so the pump doesn't burn out supplying the pressure for the home water supply. Do a Google for "water pressure tank" and a bunch of links will pop up.

I doubt if hardware stores in city areas will carry these, since they're not needed for homes on a city water system. (I've never seen them at the Lowe's or Home Depot stores here in the Seattle area.) They can probably special order them, though.

Sears sells them too. Do a search at their site for "water pressure tank" and their models will pop up.

Dave F>