When you didn't have testing done you pretty well tied the vet's hands,
and wound up costing you more in the long run.  Guess-work had to be done.
It also appears that you and the vet have communication difficulties.
That said, the vet should have known that after about 2 and a half years
in ferrets the simple formula is one ferret year equals about 10 human
years, and should have thought "ulcers" when you reported the tarry,
black stools.
 
You need these things for the ferrets' care to be best handled:
1. a vet who knows ferrets
2. no more animals than the number for whom you can afford to provide
   medical care.
 
Some shelters have fostering programs in which they provide medical care
but the people who take the animals in provide the day to day care.
 
I am not sure if I clearly indicated that the distributors are independent
of the farms/breeders.  The reason they are listed on the website is
partly up-front disclosure, and partly making the info available to those
who need to contact a distributor to acquire ferrets for a store or to ask
the distributors about stores which purchase from them in their regions.
I don't know if the distributors work on a discreet sale or consignment
basis, or either/or.  Who the distributors sell to is out of the hands of
the farms, whether it's to other distributors, stores, etc.  Of course, if
the farms find that a distributor is doing lousy work they could sell to
an alternative if one that is reputable enough for them serves the region,
or they could drop a region.  Some distributors carry only one source, but
there are those which carry more than one, and certainly some chain-stores
carry or have carried more than one source at a time, and some of those
have their own corporate "warehousing" distribution centers.
 
Hope that helps.
[Posted in FML issue 3803]