Can I level floorboards with metal or rubber washers?
Shimming a floor is the wrong way to get it level, it creates lots of pressure points that will result in cracking and more leveling problems in the future.

If the joists are unlevel, you need to sister on a new joists to the side that is level. It should extend well past the unlevel section or to the next load bearing structure.

If the subfloor isn't level, then you need to put down a self leveling compound (you'll find it with the cement in the big box stores) Take care to mark the location of your joists before putting this down, and make sure any holes are patched or blocked off ahead of time since this will flow right through them.

Also note that this will make a surface level, not flat, so it may not be the best solution if the entire floor is out of level. If you need a flat surface on an unlevel floor, you want something thicker than the self leveling compound (possibly thin set, though I have no personal experience) and a long straight edge to spread it evenly between any high points.