My beloved NES cartridges are getting pretty old, and recently some of them have been functioning sporadically.
Do you know of other backup alternatives other than RetroZone's PowerPak? It's pretty expensive.
Is it the only alternative?
Another strategy would be to find copies of the cartridge.
Try looking in local pawnshops. They usually have lots of old cartridges for next to nothing. You might have to look for awhile if you're looking for specific games.
You can also ask at a GameStop if they have any suggestions for where to look.
This site lists a bunch of backup devices, but there doesn't seem to be any listed for the NES. I don't know if the SNES devices will work for the NES.