There are plenty of different amplification systems out there that you can buy for cheap, they're composed of a microphone (generally a headset) and a speaker box. More elaborate ones will have an amplification module to help with static noise canceling and feedback.
You can also get fan-made ones that were designed for costuming purposes, with special voice effects, and that famous "static burst" sound that ends each sentence you speak.
- ROM-FX: currently out of stock, because it was one of the most popular.
- Deluxe "Pro" Trooper Voice Amp by Hyperdyne Labs: contains many different speech modes and voice effects (Boba Fett, Boussh). This is the one I use.
- Stormtrooper Helmet Sound Kit byTrikToys: comes with 2 modules instead of one, the mike amp and the effects module, 3 batteries required.
Here's a picture of my helmet rigged with the TrikToys microphone module. I didn't buy the effects circuit board because I wanted to try something a little less expensive.
Recently I switched to the Hyperdyne Labs Trooper Voice system which looks like this:
I bought a tie-clip microphone and kept the speaker amp from the TrikToys kit since I can connect it to the Hyperdyne module. This one contains static burst effects, a push-to-talk function with a remote action wire, voice effects and changers, and lots of adjustable variables. It is much more costly than a regular voice amp but the end result is much better and more fun to use.