Choosing New Rims/Hubs & Assessing Size
Freehubs can be removed from the hub and replaced; it involves removing the axle from the hub and then loosening a fixing bolt with a 12mm allen key.
Visit your local bike shop and ask if they have any thrashed rear cassette wheels; if they're cool they'll give it to you free or next to free.  Pull the freehubs and hope to find something compatible; there are several different spline patterns.