Kits 

Peeble's Originals (I Highly recommend these kits)
Radio Kits
http://peeblesoriginals.com/

Scott's Crystal Radios
Scott Balderston's website. Lots of information and he sell headphones! Excellent site!
http://oldheadphones.com  

Dave's Homemade Crystal Radio's
http://www.schmarder.com/radios/sale/orderinfo.htm


Elmer Dude
Radio Kits
http://www.elmerdude.com/ 

Borden Radio Company
http://www.xtalman.com/    

Dan's Small Parts and Kits
http://www.danssmallpartsandkits.net/

 

Modern Radio Laboratories®
http://www.modernradiolabs.com/