If your old house is on blocks like my old house you can jack up one spot at a time and place shims (cheap wood shingles) or various sizes of boards on top of the blocks (and under the floor joist). You need a huge house jack for this. Maybe you can rent one somewhere. Make sure it will fit under your house before you get it. I bought mine, used, from some house movers. But you need to know this is a dirty and possibly dangerous job (if something slips while using the jack). I did it with someone monitoring a level in the room above. If you have to level something like half your house you might want to get a pro. Usually house movers also level houses.