Any 26 inch (height) mountain bike wheel will work. The wheels you find at the thrift stores will differ in width, and consequently differ in the width of the tire they accept (Each rim will accept a range of tires). The cruisers have wide rims and big balloon tires. Replacing it with a smaller width rim, and narrower tire will work just fine. It will not look as good though. Mountain bike tires are also normally studded. Good for dirt not so much for the road. So you may also need to cost account replacing the tires (rubber). You may need to ask your bike store for help. Something like play it again sports or wally mart might be a good source for cheap tires and tubes.
Take your bike along when you shop. When you find a likely candidate put the wheel on your bike and see if you can adjust the brakes to fit the new rim. If so your golden.
Things to look for.
Take the front tire of the donor bike off and grab each side of the axel. If the axel moves up and down inside the hub the wheel is no good.
Hold onto each side of the axel and spin the wheel. It should feel smooth. If it feels rough then at the very least you will need to replace the bearings and repack the hub. I’am assuming these are not skills you currently have. At worst the hub will be beyond repair.
Make sure the hub says Shimano. There are other types of hubs that will not be compatible with your shifting mechanism.
Look at the cassette on the back wheel. Make sure there are seven rings (for your seven gears). No more no less. Cassettes can be replaced, but I’am assuming this is beyond your skill level. Also make sure the teeth on the rings don’t look to much like shark teeth. This means its worn out. The teeth should look like a stepped pyramid with both sides looking similar.