I seem to remember a thread of this recently but cant seem to locate it. Just wondered if human ones work and are these  are worth buying

If you mean just to listen to puppy heartbeats, I used an ordinary stethescope and could pick up a heartbeat quite clearly. Not sure about more technical stuff I'm afraid!

Human dopplers work on a different frequency (depth) to the ones needed for puppies - be wary of some offered to rent.
If you're interested in a canine doppler, contact Mike Brampton at Thames Medical http://www.thamesmedical.com/ - he knows the difference! :)  I've been using one of his dopplers for around 15 years very happily.

I have got an old steth but only find it works when pups are quite big. 9Alsao kills my ears.lol)I wondered about the baby ones you can get on Ebay for about £30 and wondered if they worked.