I just keep going to local yards, (found alot of 2wd ones). Craigslist, and ebay. Maybe even put out your own ad on those sites and see if anyone has one.
I was looking to buy a whole 81 -85 chevy diesel 4wd truck just to get the combo, but everyone I ran across had the sm465. I will keep my eyes open and if by chance I run across one I will let you know.
Your other option is go to drivetrain.com or others that sell them, they are pricey and then get parts at your local yard to finish if need be.