Some links to related sites and stone suppliers:

Eldorado Stone
http://www.eldoradostone.com/

Pennsylvania Bluestone
http://www.susqco.com/subsites/ecodev/pages/bluestone.htm

Endlessmountain Stone
http://www.endlessmountainstone.com/pa_bluestone.htm

Trowel Trades Supply
http://www.troweltradessupply.com

Stone Descriptions from Wikipedia:
http://en.wikipedia.org/wiki/Bluestone

http://en.wikipedia.org/wiki/Fieldstone

http://en.wikipedia.org/wiki/Tyndall_Stone

http://en.wikipedia.org/wiki/Slate