I'm heading out today to buy the rope to build the prussic knot climbing system.
Good for you! Practice the knot often, and if you need any help with it, let me know.
FWIW, although it is not the best rope in the world to use for this, and I wouldn't really recommend it. Cabela's sells a 3/8" camo rope rated at I believe 2400 breaking pounds for $15, it comes in 100ft length, I believe. Check with them and the package for all specs!