She's 6 weeks old? Who is hearing this murmur? I assume you're not taking her for another 10-14 days, so that should be time to work out with the breeder a plan of action.
I understand you don't want to not get the puppy until it's 16 weeks old the age by which most innocent puppy murmurs are gone). Nor will you (or the breeder) want to spend a lot of money on testing for a condition that might not exist anyway if you only gave it a month.
Are you near a board certified veterinary cardiologist? You might contact one and try to get an appointment for an auscultation at the 8 week mark (when you'd be taking the puppy). A board certified cardiologist will be better able to distinguish a grade I from a grade II (or III). An echocardiogram will provide the most information, but would also be a few hundred dollars, and not something I'd recommend before 16 weeks (unless it was a Gr III or higher)
If there are no cardiologists nearby, ask the nearest vet school for recommendations for a skilled internist/auscultator (this isn't a specialty, but just the skill of listening to heart sounds and ranking them). Heart sounds can be very subtle things -- training and practice will account for a lot of the skill, but some internists will have the skill even if they're not cardiologists. Some internists may have an extra skill at setting bones, some might have very good ears.
Even if the murmur has not disappeared by 16 weeks, Gr I and II murmurs don't generally indicate any problems that would cause issues for the dog itself in its lifetime. Breeding it would be questionable and dependent on future testing, but if the pup is meant to be a pet, you could take it home, neuter it and be reasonably confident that it will live a long, healthy life.
A Grade II/III murmur (as defined by a cardiologist) in such a young puppy would (to my mind) indicate the necessity of an echocardiogram to determine what's going on, and if it's anything that might progress.