Best way to stop cold air coming in from fireplace
You can build an inexpensive "plug" for the front of the fireplace.  Get some thick plywood cut just barely smaller than the opening of your fireplace.  On the back of the plywood attach thick styrofoam or the foam insulation panels cut slightly bigger than the plywood.  Attach a handle to the front of the plywood and paint any color you want.  Stuff this into the front of the fireplace to keep drafts out.