I first started off with about 250 ft each of black and tan 550 parachute cord. You can get away with much less than that if you want to just try it out - probably around 25 feet of each color will get ya started. Al looks like he is out of paracord right now in the Supply Store, but he's a good place to get some cord in shorter lengths to try your hand at one. If you want to do lots of them, then there are quite a few suppliers out there that sell 550 paracord in 250 ft, 500 ft, and 1000 ft rolls. Ebay also has paracord. Some places are limited on colors and other places have lots of colors, but they are a bit more expensive to cover the overhead of all of the colors. I'm not going to recommend one place over another, because some sell shorter lengths cheaper than others, but others sell the longer lengths cheaper that some.
Brian