Gotcha. Remember I come from the pharmaceutical manufacturing background so the only thing I could think of was process piping!
Here is one of a gazillion o-ring sizing charts out there. http://www.marcorubber.com/sizingchart.htm
Looks like an AS568-018 would work fine, 0.739" ID, 0.070" cross section, 7/8" nominal OD.
Go to your local power transmission supply house (Motion Industries, Applied Industrial, et al) and tell them you need some -018 Buna o-rings. They will probably come in a bag of 25 and might set you back $3.00.
Note: your original post said "o-ring gaskets" and I interpreted it as you were looking for actual o-rings. If you are looking for flat washer-type gaskets then that is another story. Aside from garden hose washers or pipe flange gaskets you are most likely on your own. Perhaps your neighborhood lock shop could be of assistance.