It's strange that you can't find the B-vent. Both big blue and big orange carry 6 inch galvanized B-vent in my area.
Do you have a two-stage furnace? If yes, does it run from a two-stage thermostat? Flue condensation in 80% AFUE furnaces is often due to short cycling. The furnace doesn't run long enough to actually heat up the exhaust vent and as a result there is condensation.
How close of a differential do you have your thermostat set? What is the run time-off time-run time of your furnace under "normal" operation?