Do you have any wall warts? I ask because I happen to have a whole pile of them myself, and I suspect many electronics hobbyists like myself do as well. If so, you can simply go through them and look for one that has an isolated output. I know for sure that at least some of mine do.
If not, you might be able to find wall warts cheap somewhere, like in a recycled-goods store or other secondhand store.
Otherwise you're looking at something expensive like an isolation transformer. Actually, now that I think of it, you could probably do this on the cheap. There's a guy with a bunch of circuits on the web who uses a very simple but clever scheme: he puts two low-voltage transformers (like 6.3 or 12.6 volts) back-to-back, so you get 120 volts in and 120 volts out. That would definitely do the trick. (Just be sure to follow good wiring practice, especially on the "secondary" side of the 2nd transformer.)