HP Z620: installing a third graphics card

Hi,

I would like to add a 3rd graphic card on my HP Z620. 

The HP Z620 comes with only 2x 6 pin PCI-E cables to power graphic cards. 

Any thought as to where I could get another 6 pin power connector? I had a quick look at the PSU but found no available 6 pin PCI-e connector.

Using a molex or sata converter seems like a REALLY bad idea if I don't want the whole rig to burst in flames  

thanks!