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.