What type of small non-toxic stickers can I use to tag Frogs with for Research?

I am working on a research project at Jamaica Bay Wildlife Refuge in New York City for school. The refuge wont let me use any long term tagging methods or complicated injections etc. so I proposed the use of simple small circular(square, whatever) numbered stickers to tag the Grey Treefrogs I am studying. However Amphibian skin is very permeable and I need to find non-toxic stickers to tag them with. Any suggestions? Remember I can always cut them out, I just need to find a non-toxic substance. Thanks For Your Help!