Where to Buy a Used Transmission?
Consumer Warning: Many of the “low mileage” used transmissions and engines found online for sale have fake mileage. Parts brokers use the same mileage on an engine or transmission today that they advertised and sold to someone else months ago. Brokers do not have these engines or transmissions in stock: Online brokers will wait until you order something they advertised, then find one at a junkyard, and then ship you whatever they can find at that time.

To avoid being scammed:

Avoid online brokers.
Always buy local.
Only buy used transmissions through a local auto repair shop or local licensed salvage yard

Best Option For Buying a Used Transmission: Buy through a Local Auto Repair Shop
When you go to your local repair shop, they can order a used transmission from salvage yard (either locally, or out of state, depending on availability).

Pro Tip: If you are having your used transmission installed at a local repair shop, this option is the most recommended way to buy a used transmission.

The major benefit here is that you will typically have some sort of warranty on both the parts and labor through the repair shop if the one they purchased fails.