There's an Animation Art Gallery in London, which has a wide range of stuff, from Batman to Futurama (okay, so maybe that's not the widest of ranges) and everything inbetween. I love just looking around, but everything is so insanely expensive that I'd never buy anything, at least not from there. I'm sure I could nab a good deal off ebay if I looked hard enough.
I wish I could add more to this, but I haven't had many experiences with Animation cells even though they interest me greatly.