Does anyone know where I can get the coil that fits into a coleman 7900 series furnace that will give me central air during the summer?

You do not need the exact coil Coleman would have installed. The A coils come in several sizes and are somewhat universal. This is not a home owner job since you probably lack the equipment to pumpp down the freon lines and dry them. The compressor will be outside and two copper lines will need to run to it. The compressor draws 220vac and they are available in different levels of efficiency, SEER.