"Flags of the World" (http://fotw.unis-ru.com/flags) shows four flags with shells:

East New Britain Province, Papua New Guinea
West New Britain Province, Papua New Guinea (two quite different flags, one showing a marine snail, the other with shell money)
Travancore, British India (looks like a sacred chank)
Turks and Caicos (Queen conch)

For extra credit, can anyone identify the shells shown on these flags?

Andrew K. Rindsberg
Geological Survey of Alabama