Not handy Pete but if you go to the McMaster-Carr website and enter "rubber bands" in the search window they'll pop right up. They've got a bunch of different sizes, from office type all the way up the scale.

There's only one page of them in the catalog.

I think the inner tube idea is a good one if you need especially wide bands, to do some of the things that dweste has been describing but if you need something with some real elasticity, the 1/2" wide rubber bands are a nice addition.