You can get 'branded' ones that will work. My non-MP3 Alana, has an Alpine 6-disc in the boot that came with the car when i bought it.
Have a look on somewhere like
www.edworthys.co.uk
Just out of curiousity, why would you want a cd changer if you have the mp3 option?? You can probably get a 4gb MP3 player for less than the CD changer which will give you around 1000 tracks - or about 100 albums as opposed to 10 albums max in a changer.
Although, if the CD's are MP3 converted then it will be about the same I guess?