Did you know FireWire 6-pin to 9-pin cables are the same as FireWire 9-pin to 6-pin cables? You simply flip the cable!
Using FireWire 800 and FireWire 400 Cable Converters and Adapter Plugs like the CAB-FW8-6-9-CONV and the CAB-FW8-9-6-CONV for existing cables is an inexpensive solution. However, the best solution in terms of electrical and signal shielding for bridging between 6 pin FireWire 400 Ports and 9 pin FireWire 800 ports is a 6-pin to 9-pin FireWire cable. EverythingHerePlus carries 2M (6.6F), 4.5M (14.8F), 10M (32.8F) versions of FireWire 6-pin to 9-pin cables.
Check out the full range of FireWire 800 Cables and FireWire 400 Cables at EverythingHerePlus.com in the shopping cart section on any of their FireWire Product Pages.