I just did a Google search on the term "firewire 400 extension cable" and it came up with plenty of results for them so you might just want to do that too for simplicity.
A lot of them seem to be like 10' long, so keep in mind that the FW400 spec for length is a combined cable length of 15'. The 10' ones are just a couple of random links I looked at though- there are probably plenty of shorter ones available too.
Edit: I just checked Amazon and they have quite a few- the first one on
this search results page is 6', but they have three pages of results!

(Just remember that iLink, IEEE1394, IEEE1394a and FireWire 400 all describe the same thing- a 6 pin Firewire cable).
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