Carolina Biological Supply Company: 
        Established mail order company with online catalog and great links to science sites.

     http://www.carolina.com

Parts and equipment for Amateur Scientists: 
        Links to suppliers of electronic, optic and general science materials and equipment.

     http://www.amasci.com/supliers.html

Science Stuff: 
        Appears to offer educational science products at affordable prices.

     http://www.sciencestuff.com