None that I am aware of.

Pretty much anything that you want to stuff in there will work.  You can
research what fits here:

http://www.crutchfield.com/

Their prices aren't bad either and the service is excellent.  No
affiliation, just a satisfied past customer.

I'm not particularly recommending this supplier, although I bought from them
in 2004, but this is what you need (fits with the standard BMW mounting
hardware that you already have):
http://caraudiosecurity.com/shop/product/products_id/407.html
and one of these:
http://caraudiosecurity.com/shop/product/products_id/409.html
and one of these:
http://caraudiosecurity.com/shop/product/products_id/1280.html

Works a treat! The only disadvantage is that you might have 100 or more MP3
tracks on one disc, and your head unit only displays track numbers (the
display rolls over to 1 when it reaches 99) so you can only select a track
by pressing the Next Track button. Best to set up your tracks in an
acceptable sequence and just let it play, or use the Random function.
Chris.

Alpine makes a six cd changer that is identical form factor to the factory
unit.  IIRC, it requires a converter (from soundscape or other) to match the
car's M-bus.  The MP3 cd's, which can hold over 100 songs will show up on
the display as track 1-100 and will then cycle to 01 and so forth for those
beyond 100.  You should be able to Google a description of the install.
Takes about 10 minutes.

OR, you can use an Icelink and replace the CD changer with an ipod.  More
songs, better interface and not that much more money.

R / John