There are a few people on this board who offer helmet kits. Try seraching the forum for names like sgtfang, marrowsun, bobamaker, terminal fettler, and bigkidbiggertoys. Educate your self on all the different makers and also on what helmet you wanna make, custom, prepro, esb, rotj, or jango.
A word of advice about ebay, you are better off staying away and getting a bucket from some one here. This community is a wealth of experience and information you just need to seek it out.
Use the search feature and learn as much as you can. If you still have questions don't be afraid to ask. Also do not be afraid to send someone a private message if you have a specific question about something they sell. Pretty much everyone on here will get back to you as quick as they can.
Also, check out the cargohold its the section at the bottom of the list. You can find all kinds of good deals on helmet kits there usually. I belive there are 1 or 2 helmet kits currently for sale. Go check it out.