www.nursesdirect.com has this one:
It's a Sprague-type. which I personally dislike b/o the weight. I don't like to wear watches because it's harder to wash my hands w/them. If I can't use a wall clock, I use my pin-on watch:
It's sold on the same site.
Another thing you might try is to attach a watch to the stethescope w/an allergy band.