Why do you say the stock battery is woefully inadequate?
Car batteries are pretty standardized things. You should be able to pick up one whose specifications are appropriate for your '08 MZ3 5-door just about anywhere general car parts are sold (Wal-Mart, Sears, Autozone, NAPA, etc.) The only "aftermarket" batteries would be ones designed to save weight, e.g.: AGM batts, or Lithium Ion ($$$) or whatever.
Also bear in mind that the OEM battery is not a maintenance free battery and requires periodic checking/filling of distilled water. If you're having charging issues with it it's possible that it's run low and actually lost some of it's ability to hold a charge.
Like chromal said most batteries are pretty much equal (though pretty much any batter you buy in a store will be maintenance free) with the only exceptions being light weight, small, or high capacity like the optimas. If you're running a big stereo system that's really hungry then that's might be up your alley along with possibly a capacitor and if your system is really big a high output alternator.
So what exactly is inadequate about the stock one?