There are any number of cogs that would work, from 5-speed Uniglide, to 11-speed Hyperglide. It just needs to fit the splines on your freehub. The nicer cassettes had/have little bolts holding them together that you can undo with a 4mm wrench or pair of pliers. Cheaper cassettes will need a Dremel to grind out the rivet heads.

If you have horizontal dropouts, you could probably even use a worn cog that was starting to skip when you were running it with a derailleur.