Junkyard. You can pull the axle from a newer 4R/460/FJ. Even if it doesn't have the diff you can eat the core charge from ECGS cheaper than it would be to buy a full assembly. Then just swap in the axles from your existing 8.
A lot of people will say it's a waste to lock the factory 8" rear because of the weaker ring gear. If you have the disposable funds you might be better served changing out to a newer 8.2 axle and then ordering a locked 3rd from ECGS. This was what I did.