Best place to buy Audio stuff online would be Cruthfield. (http://www.crutchfield.com)
More expensive, but best service and information.

Cheaper places that I have found...
http://www.discountcarstereo.com/
http://www.audiobahninc.com/index.html
http://www.carstereoworld.com/
http://www.usacaraudio.com/ (Annoying Midi playing)

They have great deals here too although I never did check out the ones posted above.<ul><li><a href="http://www.thezeb.com/">The Zeb</a></li></ul>