Getting a seal or any seal should be no problem, there is the I.D. inner diameter where the shaft passes through, the O.D. or the outer dia of the seal where it would press into the case, and the thickness.
A cheap dial caliper is usefull to anyone in this situation. Or just pull the seal and take it to a parts store, even though I have been in an Advance and Autozone the same day and no one in the store knew how to use a book to look up the seal. Idiots...
Anyhow it is easily do-able. Or stop in a transmission shop, they'll likely help you just so you'd come back for a trans job, find a private owned shop..they tend to be friendlier to parts hunters/do it yourselfers than franchises.