The brush you are looking for is called a sword striper. Check the local sign shops or coastairbrush.com. They sell complete HoK striping urethane kits with everything you need. Plan on practicing quite a bit to pull a decent line. I've been trying for years and still have problems going over 2 feet.
Some guys use a buegler striping tool that rolls the paint on with a wheel. I think there is a link in the links section of the board.
You can also do the taping method where you lay down two pieces of tape as wide apart as you want the line and paint it . There are other suggestions on the board. Do a search for striping and you will come up with some good ideas.