There are plenty of places that specialize in animation cels. I’ve been getting mailings from Gallery Lainzberg in Iowa (of all places) forever – http://www.firstanimationart.com/index.html . I think I even bought a cel from them.
You should be aware that many cels aren’t production cels – they were created for the collector’s market. Even though they are made just like production cels (ones actually used in making a cartoon) they were never intended for this purpose. A lot of Disney and Warners Brother cels are of this type. If they show the character is a characteristic pose, or have them really completely visible, there’s a good chance it’s a collector’s cel.
I have two Warner Brothers cels – one is a “Collector” cel, and obviously made to be one – it’s crammed with characters. That’s OK if you know what it is, and still want it. The other is a production cel of Bugs Bunny, and it’s clearly just that – he’s in the middle of a motion, it’s not whole-body, and it’s not a full front or side view. I like it more for those reasons.