As a standard, the damper throat should be the same size as the front width of the fireplace opening. As such, a Vestal 36 Inch Cast Iron Throat-Style Fireplace Damper
would be the proper size to use for a 36 inch fireplace.
Most damper assemblies are held in position with a layer of parging mix. If you have an existing damper that has rusted badly or otherwise needs replacement, it will need to be removed by using a cold chisel and hammer to break the existing parging mix loose. After removing the existing damper and any remaining mortar, the new assembly can be put into position and new parging mix used to secure it. If the new damper does not sit upon a brick ledge, you may need to construct temporary 2 x 4 supports to hold the damper assembly in position while the parging mix sets. A good parging mix to use is the Ahrens Chamber-Tech 2000 Smoke Chamber Parging Mix