for my recent build i bought my cables off of ebay.
http://www.ebay.co.uk/usr/modpcuk?_trksid=p2047675.l2559
thats the store i got them from.
wide vareity of cables and colours and they dont cost much compared to other places
the cables youll want are the 8pin cpu, 24 pin mobo, and 6 and/or 8 pin pci-e cables.