Another place where you will frequently find celluloid is guitar picks. Can just slowly amass a collection for free. Especially if they're thin, flexy, especially if they've got wavy or trippy designs in the plastic, it's probably celluloid. This can be handy for survivalism -- you buy your first ferro-rod fire starter, if you're having trouble getting wood shavings to ignite, try mixing them with shavings of guitar pick and see if the explosive reaction helps. Other folks keep a jar of cotton balls coated in petroleum jelly for a similar quick-start, or egg cartons filled with sawdust and parrafin (although that's less for getting the spark to take and more for getting a log to start burning in a stove)... and of course if you don't want to just slowly accumulate unwanted guitar picks you can buy big sheets of celluloid for like 5-10 bucks.
When we were teenagers, one of my friends lit a guitar pick on fire. It was a lot more exciting than we expected! We thought it'd just shrivel up and melt, but instead it was like the pick was made of solid rocket fuel.
Microsoft Quick Basic help was also gold.
I started up QBasic knowing nothing other than that it seemed like a thing for programming computers and programming seemed like a cool thing to do.
I typed in random words, and eventually I typed "screen". When I pushed enter, QBasic capitalized it, so it seemed important. I hit F1 and read the help. It made no sense, but the example ran and had other capitalized words so I could repeat the process.
Eventually I started making really terrible text-based Final Fantasy knock-offs.