Four words: Third Party Amazon Store.
But I get where you're coming from - I would have prefered it if Peter had snagged one or two of those cartridges and studied them to make his own formula, but him buying them isn't an issue for me. He's a smart and resourceful guy - I imagine he wouldn't have bought them directly from Oscorp and would have gone through some risk-prevention channels (ie having a dummy paypal account funded by a refillable VISA gift card, shipments sent to a PO Box, etc). It's not that hard to order things online anonymously, and I'm very surprised that people are so quick to think that Peter would be so stupid in that regard. He's all about keeping his identity a secret, he wouldn't just mindlessly order things in his name like that.