Might check with a local automatic transmission shop. We used to carry them at the dealership (GM) until the speedometer drives went electronic and the plugs weren't removed as much. GM sold them in a package of 10, as I recall. Not sure about the aftermarket, though. Maybe NAPA, too? Other than the outer o-ring, there is also a spongy lip seal on the inner barrel of the casting, which seals the driven gear's shaft.
A generic o-ring can work, but if it's not compatible with ATF, it'll get soft and start seeping in a few weeks.
Just some thoughts,
CBODY67