1) With frogs, there aren't breeders. There are hobbyists that breed, though. It's confusing. I actually recommend buying off ebay or browsing websites, as there are plenty of responsible people that have healthy dwarfs for sale. It's an extra $10 (total) for shipping, but I think it's worth it becaus you can get blondes that aren't $20 and you never get just-morphed, emaciated frogs. And contrary to belief, overnight shipping is not at all stressful on these guys. Local fish stores and local pet stores often have healthy frogs for sale. I'd steer clear of Petco or Petsmart, but if the area is clean, the frogs are being housed properly with appropriate fish (this shows that they are knowledgeable) and all look perky and healthy, go for it! Many places get stock from good suppliers, or hatch them themselves.
2) I know there is always a risk, but I've never seen it happen with these guys. Just make sure to wash your hands after cleaning the tank or feeding.
3) From a reputable source, normals usually go for around $3-$5, and blondes go for around $5-$8.
Have fun! I actually would offer to sell you a couple free of shipping, but unfortunately I won't have any till spring. Good luck!