you'll find that it is cheaper to buy a complete kit than in pieces. I got my 600watt MH kit, with that wing reflectore, bulb, and ballast for $149 US shipped. You'll find most used ballast are going for almost as much as a new kit does!
Check the USA listings, some will ship international. Avoid the kits with magnetic ballasts, thats old tech and less efficient. The digital ballasts can use either Metal Halide bulbs or High Pressure Sodium. Most magnetic ballasts are set up for one or the other, or you have to buy a special bulb ( More $$$$$ )