Another lover of 54mm goodness here. TVAG has already given you some splendid advice, and I second his recommendation for those two sites/companies. I haven't ordered from TSSD, but can tell you the service and products available from Classic Toy Soldiers are superb. They are very friendly and helpful as well – if you have questions, don't hesitate to call them.
Another fantastic source is The Toy Soldier Company: toysoldierco.com Jamie Delson, the proprietor, is a real gentleman is always very willing to spend time with advice on how to get started in the wonderful world of 54's. He also has one of the deepest catalogues around.
As for makers and brands to check, here are a couple of manufacturers you might want to check out (all available from The Toy Soldier Company):
Airfix made some classic cowboys, well worth checking out. Be warned, however, they are a little smaller than the average these days. They also made some cracking Indians.
Dulcop: an Italian maker. Some of the poses are a little odd, but they have a charm to them. They made cowboys and, if you're okay stretching eras a bit, a superb Zorro and Mexican opponents, which might be the best sculpt of the Masked Man in any scale. Really superb.
Reisler: made the best Banditos ever for the scale. Lovely sculpts and a joy to paint. Their cowboys also look nice, though several are prone, which I don't care for personally.
Paragon: some excellent Apaches and Cavalry figures.
Britains: if you like Mexican bandits, be sure to check the current Deetails tag at the Toy Soldier Company. The original Banditos are available unpainted for quite a good price ($18 for six). They are MUCH cheaper than the factory painted chaps, and paint up very well.
Jean Hoefler: made some of the best cowboys and Indians around, in a rather charming, cinematic, style. The Indians in particular are expensive, but they have tons of character. I recommend them highly.
Austin Toy Soldiers: you can find these on e-bay if you type in Austin cowboys or something similar. Simply amazing sculpts. dead cheap, and one of them is a spitting image of Clint's Blondie. There's also John Wayne and several other familiar faces from the big screen. Don't miss these. auction
Replicants made some nice, unusual types, including bounty hunters, saloon types and Pinkerton men. Best source for these might be: link
As for rules, take a look at Desperado as well. Dead simple and loads of fun for players of all ages: link
Best of luck with your search. Keep us posted how it goes, and welcome to the wonderful world of 54's!
Edit: Here's a link to the Britains unpainted Banditos: link Also neglected to mention that for the month of May The Toy Soldier is offering free shipping. Might be of interest!