If you find Helium to be a little too heavy, and hydrogen to be too risky, then go with a vacuum. Lighter than hydrogen without the explosive qualities.
If you can't find really pure vacuum, then you might just try regular space. NASA and the new Chinese rocketry program have started bringing back canisters of space from their orbitals for resale back here on Earth. Should be pretty cheap because there's not much production costs, just harvesting what's already out there.