I can recommend Thames Medical.
http://www.thamesmedical.com/Mike Brampton makes dopplers for midwives throughout the UK but also makes ultra sound (not pictures) scanners specifically for dogs - these are not pig dopplers that are sometimes rented out and work on a different frequency and so don't work well for dogs!
There is some controversy about side effects of picture dopplers when used regularly - and sound dopplers use only 10% of the comparable ultra sound waves.
I had one for 10 years when I was breeding and could detect pregnancy from 21 days and could be accurate with numbers from 26 days. A brilliant and cost effective bit of kit for those with regular litters. :)