As far as I know, they can only be purchased through head-direct.com. You can always try posting a wanted ad here in the FS forums; considering all the well-deserved praise, I'm sure there are more than a handful here on Head-Fi that own HiFiMan in-ears.
As per the single-flange tips, as our friend s0lar mentioned, the
Sony hybrids are well received by most, and come in a wide array of sizes; the very same goes for the
Monster SuperTips. In either case, the GR07's bore may not be entirely exposed, hence altering the sound that they reproduce, to varying degrees (based on the listener). That's particularly why I'm quite fond of the HiFiMan bi-flanges, as they leave just enough space to accomodate the full bore width (refer to the picture I posted on the last page), and hence reproduce the sound without any appreciable veil/muffle.