Whoops, forgot about the 3rd pin, my fault. 
The Y-cable of Amazon probably does not fit for PCIe (reviews)
How do you know that a riser needs less than 50W? I've nowhere found infos about that.
The Y-cable of Amazon probably does not fit for PCIe (reviews)
Powered risers draw less than 50 W and are perfectly safe to use with a SATA connector.
How do you know that a riser needs less than 50W? I've nowhere found infos about that.
If you have a power usage meter and a dual PSU setup like I do that has the main PSU powering the motherboard and all of the risers and the second PSU powering the VGA power inputs, you will find each of the risers use 45-50 W.
https://bitcointalk.org/index.php?topic=1984356.msg19780697#msg19780697
BBT also uses molex and SATA powered risers for his builds and has said the same. The cable in the link should work for the 6+2 cables coming from a PSU PCI-E port. If you want a better quality one with thicker cables there is this one.
http://www.ebay.com/itm/11-Inch-18AWG-PCI-Express-PCIE-8-Pin-Male-to-2x-PCI-E-6-2-Female-Splitter-Cable-/322543875241