Try the International Aroid Society at www.aroid.org.

That's of course the best site. Some sellers have good information too; I especially like Natural Selections Exotics, and there's good information at Agri-Starts, which is more or less wholesale but has lots of pictures and some cultural information.

If you're into meconostigma aroids (the large self-headers like Philodendron bipinnatifidum and P. x evansii), then this site if for you:

http://blueboard.com/pahatan/

* It helps if you can read Malay, but there are tons of pics in it.

There is an associated forum:
http://blueboard.com/aroids/

Here is a link that might be useful: Meconostigma Aroids