Paracord is ubiquitous -- you can find it at most outdoor-oriented stores.

You should search this board for the near infinite number of threads about the different varieties and online sources of paracord. In short, you'll most likely want the "real" stuff, with seven braided nylon inner threads, which is stronger. The inner threads can also be separated and used independently if necessary.

There are a variety of lightweight ropes/string/cord that you can use for knot tying, lanyards, etc. Paracord isn't always the right choice, but it certainly is popular. (And, of course, you should never use paracord to hold your bodyweight. Buy certified UIAA climbing rope for that.)