I'm a fan of the Shure A81WS (~$70/pr), others the DPA UA0896 (~$40/pr). I have both, and find the A81WS more effective. YMMV. Either will fit the MC012s. Any w/s that's roughly 20mm / 0.75" inner diameter will fit the MC012 and most other small diaphragm mics.
If you go cheapo from Radio Shack or some such, they probably won't work very well - most inexpensive windscreens are meant more as popscreens than windscreens.
Alternatively, go cheapo but add dead rats. Search for "dead rats" here on TS and you'll find plenty of threads / pics discussing how to make 'em.