Any pet shop that sells fish should be able to
test your water for you. They probably also sell
test kits (check the expiration date before purchasing).

I would not feed the fish at all during the winter months
to be on the safe side.

Your pond may have gotten overstocked over the years
and this is why you are having water quality problems at
this point.
Do you have a lot of plants in the pond? Plants help with filtration
but usually only in a lightly stocked pond.

Some pet shops will take excess fish off your hands. It is
the law in some parts of the US to keep people from
releasing fish into the wild. I advertised my excess fish with
a local club in our area and my fish were caught and adopted
by new pond keepers.

good luck and hope things settle down for you!
kathy