^Whereas early in Brand New Day they made a point of saying he'd run out of money to buy the chemicals needed to make more.
I've happily suspended my disbelief for 40 years that I've been a Spidey fan.
That said, unless he makes his web fluid out of a mix of hairspray and toothpaste, the idea that he is always nearly broke, scrambling to make the rent, pay May's hospital bills etc, doen't jive with having unlimited web fluid.
Moreover, how the hell did he make his web shooters? I'm a decent mechanic, I have a back room full of tools, but there's no way I could make those from scratch, much less find stock parts somewhere that I could modify.
For me, the ideal retcon would a mix of things they've tried. Peter creates his own web fluid in a gland in his body. Maybe his arse, but we don't have to know. But he has no way to shoot the stuff 40 or 50 feet.
Uncle Ben was a mechanical engineer who liked to putter around in his garage. He was making a device to spray, let's say paint, to do the side of the house easier when he was up on the ladder.
After Ben dies, Pete finds his little inventions and fills them up with web fluid.
Pete is a chemist, not an engineer, and is able to use his biological fluid in experiments to make different kinds of webbing by adding different solvents. The solvents are cheap and easy to come by, but in a pinch he can run to the bathroom and refill his cartridges with standard webbing.