Contact this company:
http://www.action-electronics.com/belts.htm#Measure
There are several companies that sell NEW drive belts for virtually any machine.
RichGraf, November 2009
I don't know the definitive answer to your question Denise, but if you have the player open then take it into a hardware shop and ask if they have an "O" ring that would fit. Good luck.
Bigh, November 2009
edit
I can answer this question...
Where can I get a drive belt for a fisher price cassette player?