Take a look at this $8 cable
Official max length is 4.5m so you won't find anything longer.
Also I tried a couple firewire cables in the past and they won't plug in cleanly at the iSight end with the plastic caps to hold them snug.
Lastly the iSight is 640x480 resolution. OK for most stuff but new cams are much better.
(1) Use a 6 pin - 9 pin cable or an adapter such as one of the following to connect your FW400 external iSight:
• This Firewire 400-800 cable can work: (I use this one.)
• Some report that this low-priced alternative cable works:
• To use your iSight's FW400 6pin-6pin cable, you can use a 6pin-9pin adapter like one of these:
http://tinyurl.com/ydsbb6m (I successfully use this one, too.)
(2) For longer than the 4.5 meter maximum Firewire cable length specified by Apple,
use some kind of "active Firewire cable extender"
One example: http://sewelldirect.com/firewireextension.asp
Many are available, so search locally or on the web for what meets you needs.
(3) However, understand that external (Firewire) iSight may not play well with other Firewire devices connected at the same time. For more info on possible conflicts, see http://www.ralphjohns.co.uk/EZJim/EZJimpage6.html
(4) If you must connect other Firewire devices while using your webcam, the simplest solution may be to use a compatible USB webcam.
Another benefit of using a USB cam is that long cable runs are easily accomplished with powered USB hubs and extended cables.
The downside of USB is that all devices share its bandwidth, so you will see degraded USB cam performance (such as reduced frame rates) if you have other USB devices simultaneously working on the same USB bus.
For info about camera alternatives, see http://www.mac-compatible-web-cam.com/