Mail order is a great option, eBay or any other vendor. Another option would be automatic transmission oil, several people have reported good results with it. I know 10w works, I think 20w would be too thick but i haven't tried it. Normally I'd say try it, but if it doesn't work it's a real pain to change it. Since I don't know if it works and it's a lot of work to change it if it doesn't, I can't endorse using 20w.
Scott