I have a two story house with four bedrooms upstairs. When using the furnace the master bedroom gets really hot & the other bedrooms, which are farthest from the furnace stays cold, I have plastic over the windows & I closed all the vents downstairs except two & nothing seems to help. Do you know if can purchase some ?

Purchase what? Your duct work is incorrect causing a imbalance.

I was wondering if there was some type of mini fan that could be place in the duct to force more heat into the other rooms. You mention that my duct is causing an imbalance how do I get that corrected & do I call a heat & air company? To get the duct banlance is that major work?

I about a ajustible vent one with the levers and open and close it as needed. Also a small fan buy the back most vent will pull the air along.

The duct is probably to small or the duct dampers are not adjusted properly.