Get Trinity "BOMB1" Grey foam inserts. They are designed for 1/10th buggy tires and are actually a little oversize (so they can be trimmed to fit) but they will make your tires nice and firm.

Make sure the ones you buy are the Grey foam - the white foam is not as firm even when sold under the part number listed as "Extra Firm" (I know from sad experience).

The JConcepts grey 1/10th buggy foams called "AND-1" (or "AND1") are similar.

If you can't find them, PM me - I have some I would sell.

1/10th buggy closed cell foams might also work, but if they don't fit exactly you either have a lot of work trimming or extra space in the tire.