Option 1: Head to a thrift shop and buy either a) an older though higher end bike for cheap and fix it up if need be, or b) a similar bike as yours and use it for spare parts. You can also check yardsales, classifieds, craigslist, etc. The preferred candidate is a damaged bike like yours with at least a good rear wheel. Inspect and make sure the hub is set up good.
Option 2: Head to the local bike shop and invest in a good rear wheel. Spend a little more on something that can take a beating and last a while. As you get funds, start piecing a bike together (new frame, components, etc.) and transfer over all the parts you invested in from the prior bike.