I had a customer come to me with this problem not too long ago.  Told them to
go to NAPA and take the old o-ring with them.  They have an assortment in
stock and can match your old one.  He wrote me the next day that he was back
on the road.

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242