BMW batteries from the dealer will cost a lot because the service also includes coding.
Yes you can use a generic one but get one with the same specifications as what you have now, ie: wet cell vs AGM, 80ah or 90ah, etc. Battery must have vent tube. Coding is not mandatory but recommended.
I'd recommend talking to your local BMWCCA club for their recommendations for good local indy garages.
http://www.nybmwcca.org/
Especially with a 135, you need use a good indy garage that knows these cars well.
Good Luck
Oh, and buy a good battery maintainer from Deltran, CTEK or Schumacher.