It's been several years since I've seen one in person--is it a thin plastic wheel with a small rope groove? The places that come to mind for searching are McMaster-Carr, Essentra Components and American Science & Surplus. Also, if I recall correctly--last winter or spring somebody on here posted a link to a robotics site that had a lot of cool stuff as well.
Would take some looking around in the online catalogs, but those are the places i'd start. Good luck!!
P.S. I am not responsible for the amount of "cool stuff" you may accidently purchase while looking at those web sites!!