Recently picked up some cheap stepper motors from AllElectronics:

Epson SMT65 - http://www.allelectronics.com/cgi-bin/item/SMT-65/400/SMALL_STEPPER_MOTOR_W_6_LEADS_.html

Epson SMT42 - http://www.allelectronics.com/cgi-bin/item/SMT-42/400/STEPPER_MOTOR,_EPSON_.html

Folks on the discussion boards vary as to whether these are unipolar or bipolar or even universal steppers.

Scherz's Practical Electronics for Inventors has a good section on identifying stepper motors on pages 421-422.

This book is available online through eBrary for Stanford students:

http://site.ebrary.com/lib/stanford/Doc?id=10015286

O'Sullivan and Igoe also have notes on stepper motors in the Physical Computing book, available through eBrary for Stanford students:

http://site.ebrary.com/lib/stanford/Doc?id=10082102

For unipolar steppers, use a Darlington Transistor array:

Jameco#700630 ULN2004AN DIP

Data sheet: http://www.jameco.com/Jameco/Products/ProdDS/700630.pdf

For bipolar steppers, you'll want to use an H Bridge:

Jameco sells SN754410NE quad half-H bridge ICs:

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=1054684

Hobby Engineering in Millbrae has them as well, along with a data sheet:

http://info.hobbyengineering.com/specs/SN754410.pdf

Here's a Nuts and Volts article on a stepper motor driver:

http://www.nutsvolts.com/PDF_Files/stepper.pdf