Mendeola has been making transaxles for a long time, traditionally for the sandrail market. They have been working on launching a street car business for some time, and the SD5 is the first entrant in that niche so far. These transaxles don't come from other cars- they are specifically built for the submarket they serve.
Recently, a street version of the SD5 was launched with the help of Factory Five Racing, who managed a sort of group buy for a limited number of transaxles for their GTM kit. These transaxles were very late, and are only now (early 2012) beginning to arrive in their customers hands. So far, there are no real-world experiences with how well they work, but more data should become clear as these units trickle into cars. Only a few have actually shipped, and there are some concerns about the eventual pricing and general availability of these transaxles.
These transaxles are available direct from Mendeola. They have a small dealer network, but most of them are sand rail-focused and as such, don't yet know much about the line.
There is little documentation beyond what is on the website (above), but there is a picture stream that apparently the company is using as a sort of installation manual. Those pictures can be seen here. <apparently Mendeola has removed this documentation- we'll update this when/if they put it back>
The transaxles are so new that there isn't much data about issues. As they are discovered, this section can be updated.
Apparently the transaxle can use some standard LS-specific parts like clutches, but requires a specific starter (the SDR IMI-140).
See the link above to see more pictures.