Buyers can find used transmissions in various states of condition on UNeedAPart.com, and Rebuilt Car Transmissions Network sells rebuilt units. However, buyers should exercise caution when buying and installing used transmissions.
Used automatic transmissions sometimes have significant problems that eventually require replacement, so the money saved using them might be wasted in a fairly short period of time. Rebuilt transmissions are another option; they typically use replacement parts to restore them as closely as possible to like-new condition. However, the labor required to rebuild a transmission drives their cost up significantly, and they often don't come with a guarantee. Adventurous buyers might even want to consider buying a transmission at a scrap yard, although there's no guarantee that it works at all.
Manual transmissions are often reasonable investments due to their simple design. Most of the mechanical components of manual transmissions easily survive through the typically life of a car, and clutches are fairly simple to replace if needed. Automatic transmissions, on the other hand, are much more complex and can be difficult to inspect. When shopping for transmissions, it's worth noting that the labor cost of transmission replacement is significant; buying a used transmission doesn't affect the amount of labor required to install it, and replacing a transmission safely often requires expertise.