My right rear floor pan has rusted through. I cant seem to find a replacement

lift up rug cut rusted area out,take some thin metal or metal sign cut to fit then weld in ,put silicone around top and bottom you can even weld from bottom but put silicone on bottom of car then undercoat heavily . you may have to take out seat be careful welding inside car . this is one of the waysyou can fix it yourself ue a hammer to form metal as you weld .

have a good body man look and see if its repairable to start with,then get est