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Think you're referring to a simple off shoe camera cord, for the Canon system it is the OCE3, not sure with Nikon. They are priced in the neighbourhood of $100 new, maybe a bit less. But a quick search of Ebay will turn up tons of cheap third party alternatives that go for less than $20. If you have a popup flash then you can also set it to master and have SB-900 act as slave and you can hand hold it or put it on some kind of bracket. Then there's the PC sync cord option. Probably the cheapest though there's usually an upper limit as to the speed of the flash allowed. But these go for under $10. If you want to get into radio popper and unattached category then you're going to start shelling out money. Look at the sticky thread in the macro forum showing people's setups to get a good idea of some options.
Hope this was helpful,