Welcome to the forum. I didn't have luck Googling for a plunge base, the manual doesn't mention it as an option.
Normally bases aren't universal, there might be a plunge base for another router that fits, but I don't think it is worth buying new, only try it if you can get a used one cheap and are confident the diameter is right. Take your router out of the fixed base, close the latch of the base, measure the diameter across the top, then find another router model with the same diameter.
What router projects do you have in mind? People that use routers a lot often have more than one, you might keep using your Craftsman with the fixed base and buy another model with a plunge base.
Normally bases aren't universal, there might be a plunge base for another router that fits, but I don't think it is worth buying new, only try it if you can get a used one cheap and are confident the diameter is right. Take your router out of the fixed base, close the latch of the base, measure the diameter across the top, then find another router model with the same diameter.
What router projects do you have in mind? People that use routers a lot often have more than one, you might keep using your Craftsman with the fixed base and buy another model with a plunge base.