Walmart - Energizer Max, part number 108-94R, $117. The book that hangs off the shelf where they keep the batteries lists it for for the various model years of E46, including the M3, and doesn't differentiate between coupe and convertible. I have a convertible, and it fits just like the stock battery. Works great so far.
Here's a guy on Rennlist that also uses it, he may be able to give you some long term feedback.