I usually buy deck line and bungee cord at Topkayaker.com click on shop and select category...They seem to have a lot of kayak parts for "one stop shopping".

They have 1/4, 3/16, and 1/8 bungee cord. For deck lines I use the 4mm or 5mm reflective line. Its more expensive but hopefully in low light situation it could increase my safety, besides the design pattern look cool on your kayak.
K.Greer