I met a guy passing through our town on a 26" bicycle with a flat. Unfortunately, our town doesn't have anywhere to buy bicycle parts, and I had nothing better to do that day, so I walked my bike alongside him all the way to the next town to get a tube and help him fix his bike.

Unfortunately, once we got to the Walmart, they didn't even have any 26" tubes. The next best thing they had was a 24" tube. I looked at the dude and said screw it, it's rubber, it'll stretch. So he went ahead and bought that.

It's not quite easy to make a 24" properly fit into a 26" tire, but I found that if I pre-inflate the tube until the diameter stretches out close to a 26" size, it can be installed, though not quite easy since the tube already had some pressure in it by then.

I wouldn't recommend that for everyday repairs, but in an emergency, sometimes you just gotta go with what works. Now what you're saying is that the tube you have doesn't even pack all the way into the tire, yeah that's not good at all. If anything, if you can't find the exact right size tube, I'd go with the next closest smaller size instead, it definitely can work.

Good luck