We get most of our helium-3 from decaying tritium right now (a lot of it in the US comes from tritium present in nuclear weapons). The real question should be: where does our supply of tritium come from? Right now, the vast majority (all?) of our tritium supply is bred by irradiating lithium-6 and lithium-7 with neutrons. This process is currently significantly cheaper than space travel to mine He-3, but the demand for He-3 is also very low. If some technology were developed to dramatically boost the demand for He-3 (say, fusion power), and space travel became dramatically cheaper, then at some point, it might be economically feasible to mine the moon for He-3.