Yeast starters aren't usually required if you are pitching a typical 10 or 11 gram dry yeast packet on a five gallon batch of wort. The amount of yeast in one packet is more than is needed to get your 5 gallons of wort off to a good start. For a 10 gallon batch, just pitch two packets.
Yeast starters are most often used to increase the amount of yeast from a liquid yeast pack before pitching, since there will be far less yeast in the liquid pack. The major liquid yeasts in the US come from Wyeast and White Labs.
Wyeast has Activator smack packs with 100 billion yeast cells, and Propagator smack packs with 25 billion cells. The Activator packs don't require a starter for a five gallon batch, although many people make one anyway. They are designed to be directly pitched in five gallons of wort. The less expensive Propagator packs are intended to be used in a starter to build up the number of yeast cells before pitching.
The White Labs vials are also designed for direct pitching in a five gallon batch, but White Labs recommends a starter if your starting gravity is 1.070 or higher.
You'll probably find the yeast companies web sites very interesting, and you can learn a lot about yeast from them. They provide a great deal of information geared toward home brewers, with Q&As, yeast profiles, and yeast starter instructions.