One of the most commonly used rod leashes that anglers buy are those from Sea-Air-Sports, and they cost about $20.00. It comes with a brass clip on one end to attach to a D-Ring or a Pad Eye on your kayak, and a velcro loop that goes around the fishing rod itself. The velcro loop has a quick disconnect feature so when a fish hits, you can easily disconnect the rod from the leash for maneuverability if you need to.
Malibu Kayaks Extreme fishing kayak
Some of the other brands of purchased fishing rod leashes are made and/or sold by Surf-To-Summit and Crack-Of-Dawn Paddlesports. Alot of fellow kayak anglers make their own rod leashes from strings, stretch cords, phone cords and brass clips with good results also. The main thing to realize is that if you flip over and your rod(s) are not secured somehow to your kayak, there is a very good chance you will lose your fishing rod setups. And that can run from $40.00 for a cheaper setup to $600.00 for a few good quality rod setups.