Pulley? You mean a cam? Most bow and crossbow parts are not interchangeable. Normally, the cams are designed for a specific limb. Unless you can find a Barnett dealer that has that cam in stock (which normally they order then directly from the manufacturer), I think you are SOL because they update models every year now.
If the crossbow is over 6-7 years old, I'm willing to bet you will have a tough time getting parts for it. You might want to look on Ebay and see if you can get an cheap one and part it out. If you need a new string and cable, a good bow shop can make a new one if they can't find the exact one that fits your crossbow.
I've never worked on a crossbow, but I assume you have to have some sort of press to load the limbs to put the string and cable on. Translation, you can't do it yourself unless you have a press.