where do most people buy replacement batteries?

[btownguy]

I'm going to be opening up my MBP to replace my 5400rpm HD with a Seagate Momentus XT soon. My battery could stand to be replaced as well so I'd like to do it at the same time I replace the HD. I don't want to leave my MBP with Apple to have it done so where can I just buy the correct battery myself?

 

[wywern209]

i believe ifixit sells batteries.