You have lots of choices, and I would recommend doing some research into the different sets you can get...
Tom's Modelworks does basic sets designed for the intermediate modeler. The parts are single etched and easily manipulated. Tom's prices are very reasonable.
Flagship Models does very nice brass sets, although the product range is not quite as large as these others. The parts are relief etched and very affordable.
Gold Medal and White Ensign Models do some of the finest and most intricate photoetch you'll find. I recommend these companies if you're experienced with photoetched parts and really want to take your model to the next level. These sets cost more than the previous two, but you get what you pay for.
Tom's Modelworks can be purchased through Squadron-Signal. Gold Medal and White Ensign can be purchased directly through the manufacturers, and both Loren at GMM and John and Carolyn at WEM are great folks.
Trident Hobbies and Pacific Front Hobbies all carry these sets. I'm sure there are others out there (forgive me if I've missed anyone) but these are the folks I normally deal with.
I would strongly suggest you visit www.steelnavy.com and read the reviews of each of these sets. They offer previews of the frets as well as parts breakdowns. Most of the time, they scan and post the instructions so you know in advance what you're getting into.
Hope that helps,
Jeff