Partsexpress.com in the number one source. Go to thier website and order a catalog. Also check out their tutoirals and stuff. They have a ton of stuff about DIY speakers, and a lot of partially built kits.
www.speakerbuilder.net has some low cost designs using the classic Dayton line from PE and some good articles on speaker building. The DIY forum at www.htguide.com has some designs using the Dayton RS line from PE. It's a jump in price, complexity and sound quality. Not that the classic line is bad. They are better than most speakers in Best Buy or Circuit City.
On a side note, why not snag a kit from one of the above mentioned websites? The only reason I say this is that I learned more from building a kit than I did from hacking it on my own....Once I finished one of those kits, I ended up being pretty confident with the whole building process, and on the plus side I ended up with an excellent set of speakers! It's very difficult if you have no prior knowledge to 'R&D' your own speaker from scratch...hard enough when you do know what you're doing I find :mad: (even though i'm far, far, away from being any kind of good at this). Also generally speaking you get great support from the websites themselves should you have any difficulties.