Geckodrives seem to be a good compromise in terms of price, such as this one:
http://www.geckodrive.com/geckodrive-step-motor-drives/g250x.html
You may find cheaper stuff on eBay but caveat emptor. Watch out for "marketing
amps" - any device based on a single chip will be unable to drive 3A in practice
whatever its datasheet says. Discrete MOSFET H-bridges are required to implement
a chopper-drive that handles 3A.