I purchased an old 80's Pro Cycle 24" BMX bike at a garage sale, everything is there except the wheels. I'm assuming the origonal wheels would be rare and hard to find, correct me if im wrong. Are there any alternative good cheap wheels I could put on?, say from a 24" mountain bike, and change sproket?

You could pick up a good used set of 24" Araya 7X laced to loose bearing hubs for about $150 on ebay.

Danscomp sells Sun Superstock wheels in 24" for $150. You'll still need a freewheel. Shimanos go for less than $20, or can find a place that sells White Industries freewheels. Whites can usually be found for around 80 bucks online, but I've seen them as low as $60.

Just don't get an ACS.