If you want a short single cable just buy the cheapest decent brand CAT5 Cable you can find (Belkin isn't a bad brand cable and can generally be found pretty cheap).
If you are wiring an office or home and you want a ton of cable, I would buy a 1000" roll, connectors, a crimper, and a cable tester. If you are wiring your home, I would check the difference in price between CAT5 and CAT5e as it may be worth going CAT5e in the long term.
One can make cables that will work fairly easily (with a little bit of practice and good connectors and a good crimper make all the difference). Making cables that will successfully operate to the CAT5 specification and pass a commercial CAT5 cable tester is much more difficult. One has to read the CAT5 specification and follow all their guidelines about how much of the cable can be stripped on each end (too much stripped adds noise and won't be as fast of a cable), how much of each wire needs to make a connection with each pin, and how to choose cable lengths. I would not buy a cable from someone who did not know and practice these things.