There are at least three aftermarket options available for the Prius. The vendors' sites:
www.vaistech.com
www.factoryinteractive.com
www.coastaletech.com (caution: there have been some customer service issues reported with Coastal, though not universally experienced)
I don't know if they've been discussed here, but they've all been covered in length on PriusChat. You might consider searching there for more. I have the Vais unit, and I have been quite pleased with it. See
this PC thread for details.
As I describe there, I had an initial, aborted (due to a faulty cable) installation attempt made by Circuit City. I don't consider myself very mechanically inclined, but with some good instructions I found on PC, I made the second, successful attempt myself.