Stan the man:

In my part of the US, the smaller hardware stores frequently don't stock tubs, but you actually should have no problems locating them at the "big chain" lumberyards like Builders Square, Lowe's, Home Depot, etc.

Any place that carries janitorial supplies - especially the heavy-duty stuff, is a likely source.

Don't overlook other possibilities. A small (pref. metal) trash can - 15 to 20 gal range - can work well. A couple of notorious noted performers around here have used old auto gas tanks (clean carefully!) to make some remarkable "instruments."

A large tub may give slightly more resonance, but anything from about 15 gallons on up will work well. Keep in mind that you'll probably want to be reasonably portable, so don't get too carried away with size - especially for a first try.

My own "classic" uses two tubs, with a tuning diaphragm between them, to produce "truly melodious" double-bass effects - but I had to put wheels on it to get it around the campground. And I never know if I'm being followed by appreciative musicians, an angry mob, or a buch of people who think I'm wheeling a keg to a party.

John