The vets I know always have several resident cats who will be called
upon to donate blood if needed. They are the pets of the office
staff, and are usually stretched out on the counter eyeing the dogs
with disdain.

BarB

I found out when my cat had to have a transfusion that they have
donor cats, either office cats that live there, or vets, techs or
friends cats that will be brought in when a transfusion is needed.
They don't store cat's blood in a blood bank like they do for
people or even for dogs. My cat contracted FeLV from a donor cat
and I found out that in my state, vets are only required to test
donor cats yearly for infectious diseases.

My vet has resident cats that sometimes serve as blood donors.  These are
office cats and are also great pets.  They receive lots of attention and
clearly enjoye their way of living.  Some shelters will also provide cats to
serve as donors.  In such cases, the shelters usually require that the
recipient's hoomin must agree to adopt and care for the donor.

MaryL