Originally, I had assumed that difficulties finding a reliable source of helium would preclude using it as a lifting gas. It turns out that this is not the case: you can usually purchase small helium tanks for balloons at a party supply store (e.g. a Michael's or Party Time where I live). The current cost is approximately $4/cu ft. at retail. 3 cu ft. is roughly equivalent to 85L. The helium supplied this way is mixed with air: "at least 80% helium" (presumably 80% then...). This will reduce the buoyant force accordingly. As a next step, I'll call around to see if industrial gas suppliers can provide tanks at a better cost. In the meantime, I still haven't figured out how to heat seal seams in those foil space blankets.