anywhere that has plumbing supplies should have teflon tape. but if its the same teflon tape i use, it isnt sticky. its kind of like a thin plastic strip of plastic that has a thickness somewhere in between saran wrap and a disposable plastic tablecloth. we use it to wrap around faucets to help seal them (so water doesnt shoot out of where they're connected). its safe, but i cant imagine what you could actually use it for in a newt tank. what are you trying to tape?
i cant imagine duct tape would be ok in a tank. that stuff even smells bad, probably from the chemicals in the glue.