I got it from a online BMW parts website that's very well known on here (I'm not sure if I'm allowed to say websites here) bu the part # is 01990037287 and its listed as "DVD ROAD MAP NORTH A" for $11.56 and most likely say 2011 but 2011 & 2012 update dvds have the same exact part number. I suggest you email them and have them make sure that the part # comes with and they send a total of 6 discs (1, 2, & 3 of 3 for 2011 and 1, 2, & 3 of 3 for 2012) and the discs are labeled "CIC Premium" & "HIGH".
That's step 1. To complete the update you'll need a VIN specific FSC code. I got mine from a guy for $97 and received my code 5 mins after paypaling him! PM for that info.
The update process takes about 2.5hrs and you change discs at 35% & 75%. After you insert disc1, it'll ask for your FSC code. You enter it using the idrive knob and the update will begin after its entered. You can update as long as the cars on so that means you can drive, listen to the radio/iPod or leave the car on but motor off. You can also turn off the car mid update and it'll save your spot and resume when the car turns back on. If you're on disc 3 ans turn off the car, it'll ask you to reinsert disc 1 so dont panic lol. Just insert disc 1 then a couple secs it'll ask to insert disc 2 and then for disc 3 and resume from the last %. The last disc will automatically eject on its own after the update is complete then it'll ask you to accept the Nav restart. All done! You can verify you now have the 2012 maps by clicking Options in the Nav screen then scroll down to Nav Version on the bottom.