If you type in Littmann stethoscopes into the Altavista search engine, you'll get a listing for a site called Allheart.com. Littmann Cardiology III for $117.00 - Master Cardiology for (I think) $137.00.
Seems like the best price out there.
I'd really recommend against using Allheart for anything. We ordered scrubs through them for a club sale, and it was a disaster. They completely messed up the order, the scrubs arrived close to two months late, the material was very, very low-quality, and they are _impossible_ to get in touch with via phone or e-mail. They really didn't care at all about our problems, and did very little in the way of recompensating us.
You obviously wouldn't have to worry about some of these things when ordering a stethoscope, but I'd still be cautious about ordering from a company with such poor customer service.