S.
Portable ones are nice, but the wires and trying to be careful with it while it's hanging in the car can be a hassel! Also, when we travel, once our kids have watched movies for hours on end in the car - they aren't really interested once we get to the hotel! We have 4 kids, so for me - one that is installed is best...actually we have both, but rarely use the portable.
Back to the original question, you can call some of the companies that do installations - I don't think you have to buy the system there to get the installation. My husband is in the car business and may know of someone, too. If you are still interested, email me and I can get him to see if someone is interested. They are usually willing to make a little on the side.
S.