I picked up the rubber o-rings for my DIY denitrator from NE Seal in Burnaby. The items seemed a bit pricey, but they claim to have every size of o-rings ever made, and I had a specific size that I needed ... which sounds like your situation as well.
http://www.neseal.com/
To get the proper o-ring, you'll first have to determine the specification of your missing o-ring. The following page has a sample chart with the corresponding #'s for each o-ring ... they show thousands of possibilities. The odds are that the standard AS series of sizes will match your needs.
http://www.oringswest.com/
Good luck ...