Classifieds I have seen quite a few g36 mags for sale. For quality ones, 'bulk' won't come cheap.
But if you want a box of mags for a hundred theres always MAG or STAR magazines available. They make pretty decent lowcaps/realcaps if you take the time to take care of them. I plan on getting a box of those soon for my CA36E just for backup. what I really want is a good handful of TM or CA low caps.