On some models (if there's room in the spinner, or you aren't using a spinner) you can use a brass 'spinner-nut) (large "acorn" type nut) adding the weight as far forward as possible.

There are also flywheel weights made, mostly for use by Pattern competition aircraft, requiring a 3 inch or larger plastic spinner (can fit in some 2.5 inch aluminum spinners) These are available in 1/2 to 3 oz sizes.

Sometimes just a couple of extra washers in front of the prop is plenty of weight...

The further forward the weight, the less weight you need to get the same CG change.