i've contacted a few shops and whatnot to have just a rear wheel built, but i never get an answer. am i missing something here? is it against bike ettiquette to ask for just one wheel built with new hubs and rims? i imagine mabey they would have to order a set, but i'm pretty sure level sells hubs singularly.

on the other hand, anyone know somewhere online i could get a rear built and shipped?

You can get singles of everything but spokes, with a few exceptions.
Bike shops can also get rear flipflop wheels prebuilt from Quality(A wholesaler) they come with MA3 rim and surly hub.

Bike shops should let you get just one wheel. They don't have to order a set. If you want an online place, try superspokes.com

for fixed wheelbuilding, go to firehouse at 50th & Baltimore. Good mechanics who dont get enough work. They may not be able to order levels (or they may...), but if you get one yourself and ask them to build it, they'll do a fantastic job. They once rescued a wheelbuilding mess that I made, with excellent results.