Some nice DIY speaker designs along with some articles can be found at these sites:
 A few older sites...these haven't been updated in awhile and many drivers are no longer available (NLA), but some designs are still possible and you can learn a lot reading the writeups and decisions regarding box modelling, cross-over design choices, etc.
Speaker and speaker driver measurements
Zaph Audio (old, but still plenty of relevant stuff)
Useful forums for speaker building
Software (Only free software listed. This list is deliberately not comprehensive, as very long comprehensive lists of everything on the internet don't seem all that useful.)
Crossover design
Box modelling
Places to buy speakers and components in the US
Parts Express (Dayton Audio, Peerless, Tang Band, Morel, HiVi, etc.)
Madisound (SB Acoustics/Satori, Scan-speak, Seas, Purifi, Accuton, etc.)
Newark (Multi-comp Pro, MCM, Dynavox, etc.)
US Speaker (Beyma, 18 Sound, Radian, etc.)