Welcome and good luck on the search!
I'm guessing that you're already scanning the ads here on CC and on both Cobra Country
http://www.cobracountry.com/home2.html and Cobra Registry
http://www.cobraregistry.com/ to compare prices and availability.
I'm sure the guys in SoCal can make a recommendation regarding the conversion to an AT, but you might also want to contact the folks at Hillbank in Irvine to see who they might recommend and/or use for their installs.