Kevin,

Mike is correct in the definition of "shock cord". I've picked it up on eBay before, and it's usually available at outdoor outfitters/backpacking stores.

Your deck lines are there primarily for safety - there's nothing like trying to mount a slippery kayak with no handholds. I remember taking my first boat to a three day symposium to learn basic rescues, entries, etc. - with no deck lines. It was pretty funny to watch I'm sure, but not so much as an experience.

I used 5/16" black rope for perimeter lines on my two boats. This is lengthwise nylon fibers with a woven sheath. You have to sear the ends after you cut it to prevent ravelling. I also used 5/16" shock cord (or bungee cord) in black to provide a crisscross hold down net in front and behind the cockpit. I set the system up so that the shock cords are used to keep tension on the deck lines, keeping them from sagging. I used knots on the lines to keep them from stretching too far when used for entries.

The final result doesn't detract from the look of the boat, and it's a functional safety feature.