O-Rings are usually available at any tool supply house. They are
available in sizes from extremely tiny to large enough to seal a space
vehicle, under the right conditions, that is.
In addition to using them on roll frames I've used them on radio
equipment and other machinery where obtaining an OEM replacement belt
is not a speedy alternative. Been using them in ham gear and VCRs for
many years with satisfactory results. Also have used them in a few
home-brewed projects. They are usually inexpensive and often perform
better than the OEM parts.