Nathan,
If you have some time and want to save some bucks, go hang around second hand gearbox shops and look for failed gearbox's, somebody should be able to help you out with one for a couple of R100's.
But you are going to battle to find at scrapyards, simply cause they would want to sell you the entire box.
What you could do is, if you are sure that it is the gearbox sump itself, you can take off the sump and see if somebody can repair it for you, if it is steel, then any engineering works can do it, if it is plastic, find somebody that can do plastic welding, and just have it fixed.
By the way, as depressed as you are now, this forces you to change the oil in that gearbox, which is a very very good thing, change the oil now using the right oil, and its good for another 120,000... Better this then your gearbox failing at 150,000 or something.
Good luck man :thumbsup: