Lots of places can install aftermarket seat heaters. Just had them installed in my new 2006 Saturn Vue Redline. Pretty nice. About $500 installed, included with the purchase. The Saturn dealer installed them. Seats look very good, like they haven't ever been apart.
I used to be in the automotive seat manufacturing / design business. I know what to look for in a quality seat build, and they did it right.
WET makes aftermarket seat heaters, just look for an auto upholstery shop to get them and install them for you. I would definitely have someone very experienced in auto upholstery do them for you.
Phil
2005 4x4 ext. cab dually (with superb seat heaters!)