What about this? http://www.amazon.com/CableHero-8-Pin-PCI-Express-Adapter-Y-Splitter/dp/B0021ZQK6A
It works just fine with 6+2 pin. If your power supply is stricty supplying 8 pin PCI-E power (no breakaway 2 pin ground) then it's probably a bad power supply with no foresight. My rosewill capstone 450 watt comes with 2 6+2 pin PCI-E power plugs, and I use 2 of those splitters to give me 4 6+2 plugs so I can power 2 radeon 5870's.
The extra 2 pins are just grounds, and almost all power supplies just piggy back those 2 grounds onto the existing 3 grounds of a 6 pin PCI-E power plug. It's safe.