BMW Repair: 325i Battery Replacement, vent kit, interstate battery

Question
The battery on my wife's 2002 325i has died. Two issues: how do I get it out to replace it and where can I get a replacement?  On the latter question, I took it to Sears but they do not carry the appropriate size.  

I assume this is not rocket science because there is nothing in the manual on it (did not even mention its in the trunk, let alone how to access it) so it must be obvious to everyone but me.

Answer
It is difficult to find a battery that fits in the battery compartment correctly, BMW makes a battery and so does interstate, I prefer the interstate battery it lasts longer and is cheaper and has a better warranty, allso install a vent kit which interstate sells allso. as far as installing the battery its very simple todo just hook the cables on correctly.  hope this helps   P.S. you may want to check to see if the battery is chargeing correctly and that there are no draws on the battery