Thanks, everybody! I told my big today (which was what I was most nervous about since I didn't want to disappoint her), and I'm sure she was a little upset, but she gave me a hug and told me she cares more about me than the pin
I also reported it to Campus Safety. We're going to wait a while and see if it turns up before ordering a new one. But thanks, pearlbubbles, for suggesting that it would be easiest to order it with the new members' badges. That's probably true. In the meantime, she might ask the officers if she can wear a recently deactivated girl's badge for pref, which is coming up, since it would look weird to the PNMs if one sister didn't have one. Or maybe she will wear my friend's grandmother's.
We live on a tiny, close-knit campus that is 55% greek, so I really don't think anyone would steal it, but maybe I'll file a police report just in case.