I'm so sorry you lost your pin.
Have you checked the function room where the celebration was? Or did you still have it then? If there were sisters from other chapters there, maybe one of them found it and held onto it because she didn't know whose it was.
Definitely check ebay. If you don't find yours - you may find a relatively inexpensive one. It won't be "your" badge, but it will be something. Also, check with your nationals. Someone may have found it on the ground and been kind enough to send it to them; that's what I'd do.
And if you are lucky enough to get it back, I advise that you get a chapter guard, or at least get a jeweler to look at the clasp, so you're less likely to lose it again.