Do you mean an extention for the power cable or for the PCI-E slot it self? For the slot they call it a riser card, but they cost allot.....a quick Google shows this one...
http://www.plinkusa.net/webR1PCI-E1-1R.htm
You may be better off to box in the cards in a slightly different way....
Fan in video chamber....the card or cards will expel air on there own....a side fan or slot blower sideways(there is always room beside the cards anyway) can remove the rest....Hard drive can also be in this chamber if you have room....they are not too hot
Top chamber has drives and the cpu. If its sealed tight you do not even need a rear fan as the air will have nowhere to go. A tower style heatsink is a must for tunneling air... The psu can be compartmentalized with any optical drives. Side vents or slots to let cool air in are a good idea....
You may also have good luck just modding an Antec 900 to do this....
Since the case front is all vents air will pass under the cd drive and out the psu...
You can have either a fan in the black holder or in the front of the case for bigger cards....Block off the side fan vent and the air pressure from the front(provided the seal is good) will go out the rear vents.....Hard drives and the cpu are in the top chamber.....with the 250mm fan up top the 120 front and rear....its gonna be cool....
As you see the back of the expansion card area is well ventilated....
What ever you do....post lots of pictures....