My regular advice.
Go to whitearmor.net as Splat762 mentioned. As with any costume site, READ, READ, READ.
Don't just buy armor, study it first. Learn how to build it in your mind before you even think of buying it. Know what you want for fans, audio, etc. I'm not trying to be rude or mean, but no one will just tell you where to buy armor, you sort of have to "earn" it. These websites are not a neighborhood costume shop. The worst thing you can do is hop onto the site and ask "Where can I buy armor?" When you log on, go to the BASIC TRAINING thread and read the Getting Started section, especially "Important Topics".
It's awesome that you want to join the 501st. I built my TK last September and have been trooping since last Halloween and it's been a blast all year long.
Pretty much any question you have about a costume has been answered on it's particular site. Use search tools, and just read through everything you can. Make folders on your computer for each item, save reference pics, hang out on the site and make your presence known in the forums.
Then you will find your armor.
Good luck, have fun, Troop On !