I recently bought a TV tuner for my 2001 CLK from ebay, It cost me including shipping and bank fee's to transfer the money to Germany to a private seller less than £300 which compared to Mercedes Benz £800 is an absolute bargain. I have been quoted in the region of £150-200 for fitting only because I purchached the car from new and had TV pre wiring factory fitted when built which saves alot of labour charge as these wires require removing lots of interior to fit professionaly and not end up looking like a micky mouse job.
I was quite concerned at first about buying from an overseas private seller but after alot of checking up on the seller using the ebay services it turned out to be a wise decision. The sellers are very helpful if you email them with compatability issues and technical questions.
Ebay is a good source, make sure you buythe latest version that has the 2 sets of phono connectors. makes fitting ps2 or dvd so much easier. dont forget to buy a loom as well also available on ebay.