I would suspect that for $80 at a bike shop you would get a rear wheel easily.

I just did a search on eBay and there are some sets of 26" wheels using freewheels (as the Free Spirit probably needs) for $50 plus $20 shipping for both wheels. So you should be able to get a decent rear for that at your LBS. Just be sure to let them know you are just looking to replace an old wheel, not looking for state of the art.

EDIT: Also good (especially in this forum) to list a location (not specific address, just a city and stay anonymous for safety) and if you are near someone with a spare wheel, they have been known to be offered for free, or low cost.

Also, the Free Spirit is an old department store bike, so pretty low end. If you like the ride, great! However, searching garage sales, thrift stores etc may lead to a bike with spare parts and/or a bike you like better for far less than $80... and give you a spare bike to start tinkering with.

ANOTHER EDIT: Depending on your location, bike co-ops or bike kitchens may be available to help you gain knowledge. Also 2 web sites have a wealth of information. http://www.sheldonbrown.com and http://www.parktool.com