Basically there are two..one is to try and find one from a swap meet or ebay and hope it is in good condition or that the price is right so it makes sense to rebuild it..
Or you can just bite the bullet and order a new one from Richmond Gear or maybe one of the Tremec trans..
One way makes sense if you have the tools and expertise to work with the used on and the other makes sense if you want new and good from the get-go..
Just my two cents..
I have tried most all of it and now do what is known to work..