I just did a little searching at
McMaster-Carr's website, and I think I might have found some things you might be interested in.
I couldn't find any information about wheels with keyways precut in them, but here's what I did find:
http://www.mcmaster.com/catalog/109/html/1198.html
http://www.mcmaster.com/catalog/109/html/1200.html
They aren't too cheap, and they don't seem to be keyed, so you probably won't go with them, but McMaster seems to be a pretty good suplier, generally.
Sorry I can't be of more help, right now...