Whatever club you're involved with you should ask for them to help find you a working doberman mentor. There's also the UDC and a list of breeders there who may be able to help. I don't have much experience with working bred dobermans but I'd wager anyone wanting a serious working dog will want to see the puppies in person instead of blindly trusting a breeder overseas to send them something that fits their needs.
Frankly, it is not that easy. Most of the breeders I know in Europe do not sell to strangers over the internet. That is why they rarely sell puppies to the US. The ones that do are frequently high volume breeders that can be unreliable. Do you really think a serious breeder wants to sell their best puppies to a stranger that they have never met and know little about that lives 6,000 miles away?
You will be better off searching within the US. There are good working Doberman breeders in the US if you search for them. It will be a lot less risky and you should be able to develop a mentor to help you with the puppy. A good place to start would be, UDC Breeders – United Doberman Club.
You should also know that these days, partially due to the corona virus, the demand for quality puppies is very high and you will likely have to get on a breeder's list and wait at least a few months or more before a quality puppy will be available.
What sport do you plan on working the dog in? Do you have experience with working dogs? Are you already connected to a club? Where are you located? Why a Doberman? There are some good breeders here and in Europe but these questions can help direct you?