check out this forum:
http://www.network54.com/Forum/11341
it is IMO the best one you'll find for dobes, the people are extremely knowledgable and experienced. They've helped me through a few problems with my boy!
I'd check thoroughly for the health of the pup you choose. Make sure both parents have been hip scored and have good scores (ask your vet, and there are many website explaining hip scoring); also both parents should be clear of hypothyroidism, as should their parents and grandparents if possible. The further back the pedigree is clear of any problems the better.
Also the parents should be checked and cleared for cardiomyopathy - this is a nasty, but thankfully not too common, heart disease. Dobes aren't the only ones of get it, but they get it more than other breeds - once a dobe has been diagnosed, their lifespan is likely to be very short - we're talking a couple months, maybe more if you're lucky, so do make sure the parents are clear. The test is done with either an echo or holter monitor - the latter is better I believe - it's like a techno dog coat, the dog wears it for 24 hours and all heart activity is monitored for signs of a problem. Having a vet listen to the heart will not do the trick, make sure those parents are tested!
Also, wobbler's syndrome (spinal problems) and I think haemophilia should be clear from the parents and grandparents (although I don't think it's clear if wobbler's is hereditary or not). best to be sure.
Avoid anyone whose litter was an "accident" - either they are too tight (or ignorant) to neuter their dog, or they just wanted a litter for the experience of it. Also anyone who believes a dog should have a litter to be healthier, or because they want Poochy to have a friend. Chances are it'll take you a while to find a good pup, but it'll be well worth the wait.
One last thing - avoid breeders of "white" dobes, these people have no interest in the good health of their pups. If they did, they wouldn't breed albinos.
good luck!