All extension cables from anywhere for anything will work, they have 1:1 industry-standard pinouts meaning that the connector pinout on one end equals the same connector pinout on the other end..The one shown in your photo will work fine..Wherever you got your custom cables should also have extensions available in various lengths and color schemes.
 
The solid 8pin of the extension gets plugged into the 8pin or 6+2pin connector coming from your PSU and the 6+2pin end gets plugged into you card..A 6+2pin connector will work as either an 8pin (using the 2 extra connectors) or a 6-pin (with the 2 extra connectors hanging around doing nothing).
 
Also, a solid 6pin connector will fit/work in an 8-pin socket (at reduced power) and a solid 8pin connector will fit/work in a 6pin socket with the extra part hanging out over the edge, providing there's room for it.
 
Silverstone Extension
Rosewill Extension
CableMod Extensions