Try using
www.google.com to do a search for "interchangeable bead rings". When you get your results, be sure to take a look at some of the directory sites that come up.
If you want larger quantities, try a google search for
"Wholesale interchangeable bead rings".
I found at least one supplier whose website says they sell to glass bead artists.
Another good resource would be the magazine Lapidary Journal. I think you'll want the actual printed version, not just their web site, but if you can't find one locally, you can check out their web site: http://www.lapidaryjournal.com.