The latest version (2.1, also a Universal Binary) is only available as part of Final Cut Studio, same as the other Studio apps.
However the original release of Motion 2 was a a standalone product. So in theory you might still be able to find a boxed copy for retail somewhere, where they still have old inventory.
If you have a copy of any of the old Studio apps, then have a look at the crossgrade to FCP Studio
- the prices are pretty good considering what you get in the box.