There is definitely a problem here. Tbh, this question is the reason I found this Forum; after watching the scene where he sees the promotional video then suddenly had a box of bio-cable cartridges I asked all these questions to myself. No answer is self evident, he is too poor to be ordering cutting edge, state of the art industrial materials, I think the estimated prices given here are WAY too small. The idea he stole it is a possible explanation, but given the levels of security it at least demands a scene to show this. The most plausible explanation I think is that Gwen or Dr Connors let him have some,given their level of clearance and friendship with Peter. He probably just needed to give them some guff about how much he LOVES science

This still leaves us with the bigger problem; how does he continue to produce the web fluid?
A) he continues to buy/steal from Oscorp; obvious problems concerning expense, traceability etc.
B) he starts his own radioactive spider farm in Aunt May's attic. Serious start up and running difficulties here.
C) he secretes the organic component from his own body. Not implausible given all his spider DNA, but we are wandering dangerously close to the Raimi heresy here. Of course it doesn't need to come conveniently from the wrists as he is loading the finished products into shooters. Maybe Gwen Stacy will help him produce an inexhaustible supply



t:
Building 'from scratch' is what should happen, however now that Webb has gone down the organic route it will be difficult to justify Peter somehow producing an inorganic version using his Salter Junior Chemistry Kit where the whole of Oscorp has failed, even with the McGuffin of 'Decay Rate Algorythym'. After all, that was for organic materials too. He would need to go down the route of B) or C) to maintain any kind of plausibility.
I'm guessing A) as Webb seems to love plot-lines around Oscorp, that's if he even feels the need to address the issue.....