I think you're looking for a "zip tie" not a "zip line." A zip tie is a little piece of plastic that you circle around or through something and then put it through a hole on the end of itself and it locks. They're also called cable ties.
I posted the same question a few weeks back and someone was nice enough to let me know.
I got some at
Walmart before our cruise on the 16th (which by the way was WONDERFUL!) They're very inexpensive. I think there were 18 or 20 in the pack for less than $2. They were in the automotive section.
I ended up not using one on our fish extender. No one bothered it, except that we did "lose" the letter R on it!

Go figure. It was no where to be found on it, in the pockets of the FE or on the floor. Luckily I brought some extra letters with me just in case

and it was easily fixed.
I did, however, find that the zip ties were perfect for locking our luggage!
Here's a pic of one....