I got mine at a marine supply store. (West Marine) It seems like alot of people have had good luck finding it at them. I got 10 feet for each seat for about $10 total. You will need some sort of attachments also, I used zip ties from the Depot, but you could use something else. Check out other threads for install photos! Hope it helps! 8)
I ended up purchasing the bungee cord kits from HA. What I liked about them were the two little adjustable "clips" or "hooks" which came with each cord. This allows you to keep the additional extra bungee lenght tucked in the flap at the bottom of each seat back. Then if you want to put something really bulky in the "net" you can just adjust the amount of bungee used in the net.
Check out the kit on HA's web site and you will see clearly what I am speaking about. Looks very tidy, when installed, and is adjustable too.
I picked up 1/8" bungees from Boaters World for $.20 a foot, $4.00 total. I got 4 compression cord ties from Sports Authority for $2.00.
One suggestion! I had the store cut the cord. First they wrapped it with masking tape and then cut it with a heated knife. It melts the fibers and keeps the cord from fraying. Otherwise you get a frayed mess. If you cut them yourself I would definitely wrap it first then maybe a quick pass over a flame might sear the ends.
I elected to skip the first loops on the driver's seat. That pocket will hold my topo maps and using all the loops was more pocket than I needed. The rear one holds the toolbox and first aid kit.