I've been collecting and making dollhouses and miniatures for decades, but I've never seen pre-cut inserts. You can buy wood similar to the wood the house is made from and cut it to fit the inside. Do you know anyone who's handy with tools who could see the actual house and do some measuring?
Once you have the wood cut to size, start by gluing the first floor wall (or walls) in place. (use a good glue made for wood) Once those are completely dry, then you can put in a second floor by putting glue on the top of the walls and on the inside of the house where the floor will touch. You'll have to hold these pieces in place until they are completely dry or they will move and slide. When it is dry, add some more glue just to be safe. Wipe off excess glue with a paper towel. Then you can start to add the second story walls.
If you have a miniature shop near you, you could bring it there and ask for help, or they might be able to do it for you, or know someone who can. I don't know what they would charge for this service, but no harm asking for advice. Either way, depending on the shop, they might have the kind of wood you would need since some hobbiests make their houses from scratch, rather than kits. Try googling dollhouse miniatures, also.