You need to observe the voltage ratings of the wire. In any fluorescent or halide based system, the wires on both sides of the ballast are exposed to a minimum of household voltages, 120v. Depending on the design of the ballast and lamp type, it may be many times that between the ballast and the lamps.
You should be using wire rated for 300 volts minimum. Speaker wire will work in most cases, but it is not rated for 300 volts. That doesn't mean it can't take it, but since they didn't rate it or test it for that voltage you won't know. It's not the best way to do it, and certainly not something I'd recommend.
You can get hookup wire (also called "appliance wire") from a number of different places, such as MCM Electronics, Parts Express, or Jameco. They all carry it in a variety of different sizes, colors, standed/solid, and voltage ratings.
18 gauge is sufficient for any flourescent wiring and halides up to 250 watt. Above that, you should look at upsizing to 16 gauge. 16 gauge wire will safety handle up to 13 amps at 120 volts, which you would only exceed in a very large lighting system.
Use solid wire for a neat job, but use stranded for any place where the wire flexes frequently, such as when a hood is opened or closed. If you want to use stranded and you need to use the push in type connections like lots of sockets have, you can strip the wire, twist the strands tightly, then coat the end with solder to "tin" the ends. That will make them push into the sockets easier and make a solid connection.
