How should I store leftover uncooked lasagna sheets?
Anything airtight is fine. If the sheets will fit in a Ziploc, that would be perfect. Another option that would certainly be big enough is a bread bag, well sealed. If you still have the box the pasta came in, you can put the bag inside the box to give the pasta a small measure of protection against breakage.

I checked with Still Tasty and they didn't have anything to really add to that.