[A4] CD changer

[sub]

Hello all,

I have the 2001 A4 with the symphony stereo, however I do not have the 
cd changer in the trunk..

My questions:
Is it a specific changer that fits in the left side of the trunk?
Where can I get it ?
How much is it ?
Will the dealer want 2x the normal price of a changer ?
Can the changer play mp3 cd's

PS: I don't know much about car changers so all info welcomed

thanks