Lots of places to buy new, but if you have a local shop, you might see
if you can dig around the junk pile for a bmx wheel. Two things to keep
in mind if going this route, which you may know but just in case--there
are two sizes of 20" wheels 406, and the less common 451. In addition to different rim sizes, 20" wheels can have different axle sizes. The Crust fork uses 9mm.
Bigger ones
won't fit. I got a pretty cool wheel for $5, including a usable tire.
It's overspoked as bmx wheels can be, but nice hub action and chromed
aluminum rims. Bling! Now I just have to put the thing together.