Nautilus shell? (Field Research community centre bundle)

According to the official Stardew Valley wiki:

Note: The bundle wants the shell found by foraging from the beach in the Winter, NOT the one from the mines/geodes.

I can't find anything that mentions what the drop rate is, but searching on days where the Fortune Teller says you have a little extra luck may help!

If your friendship level is high enough, Demetrius can also gift you a shell in the mail, it was how I managed to complete the bundle in Fall. (However, gifting seems random, as one time, Pam gifted me cooking items but I restarted that day and had the same letter but it contained a battery)