Go to a store that sells rock climbing gear like REI and tell them you need kernmantle rope. Check the tag on the end of the rope, it will tell you the safety rating. The rope will be dramatically over-rated for one person, but don't buy anything smaller than 3/8". The wider the safety rope, the more friction that is created if you fall. This results in less slippage when the prusick bites and reduces the overall length of your fall.

Just as an example...for fire department rope rescue a one-person rope must be rated at leastfor 4500#, two-person 9000#, etc.