Here are some companies where you can purchase drivers, crossovers, speaker kits, and other accessories: ACI, Crutchfield, Madisound, Meniscus Audio, Parts Express, Sound Clearing House, and Speaker City USA. Other companies can be found on the net, just look around!

All my project links went webdark, so maybe some day when I have the time I'll scout out some existing links on the internet and post them again.