Good luck fixing it. Hope it works out. A search should yield a lot of threads on this forum from people asking the same question about parts to repair a mower transmission.
Just replacing a seal may not solve the problem for long. A lot of times on those mower transmissions, from frequent use, moisture has gotten into shafts etc., and caused rust. Or, wear has occurred on a part that only comes in a subassembly where the subassembly costs as much as the whole thing. I might consider cleaning it up well then watching to see where it is leaking from. One solution is to run over to NAPA and get some UV dye. It is easy to spot with a UV light. I found a couple engine leaks on a car restoration that way - added the dye and used a UV flashlight in the dark. The source of the oil showed up as a bright glow. Found leaks that I did not know even existed.