I just ran into this same problem a month or so back and it took some effort to find the proper-sized O-ring. I was going to order one from an O-ring manufacturer, but they have a $20 minimum, and I couldn't see paying $20 for a single O-ring. I eventually found one in the plumbing section of a chain hardware store called Orchard Supply Hardware. Seems to be working quite well.
However, the other day, I was in Pep Boys and saw that they sell a plastic box filled with assorted O-rings, including one that looks like it might work for that particular oil pump seal. The brand is Dorman, and the O-rings appear to be off high-quality, high-temp material.