AKM V2 Construction 43

I suppose this works. If you can't find a spring, you can make one of the right length from a spiral notebook.

The only problem is that it might be too stiff to actually allow the bolt carrier to be pulled back, or it might flex upward and push the dust cover off.

The real AK spring has a few interlinking metal rods on the inside, which keep it fairly straight, and they can slide over each other when it's compressed. It's also much longer and narrower, and really unsuitable to being replicated in papercraft.