Did a quick bit of research and found this, hope it helps.
If it's a Miroku reproduction 1892, parts can be tricky. Try these:
Numrich Gun Parts (gunpartscorp.com) - best bet for Winchester-pattern extractorsBrownells - carries some 1892 partsVTI Gun Parts - specializes in older/replica firearms
You might also find luck with Midway USA or contacting Cimarron/Uberti directly if it's an import model, they sometimes cross-reference.
Worst case, a gunsmith can often retrofit an original Winchester 1892 extractor or fabricate one. The caliber helps narrow down compatibility.