OldIHC

is 98680h a good number

Good number for what?
The cab mounts are not all the same for all models. What size is your truck? 120, 110, 150, etc.

r 100

need rubber mounts to set cab on frame for 1954 r100

The front mounts are 110 057 R1 and 98 680 H. The rear mounts are 98 680 H and 100 678 R1.

This is what the Canadian front "L" cab mounts look like. The rear mounts are little more than a rubber separator, a big spring and a long bolt. Do not bolt the cab firmly to the frame. the flexing will destroy the cab.
I have cabs from 150 trucks. The rear mount is in the center of the cab and there is a cross-frame suspension piece. I am trying to get it to load, but this forum will not allow it.

now where do i buy them from ?

There a zero chance of buying a new cab mount set. Maybe you might get lucky and someone on this forum could have a set for sale. I do not know if I have anything for sale, just yet. Best advice, don't wait for me to find one and send it. I will call my friend MAGIHC MIKE and ask if he has a set.

How good a welder/metalworker/fabricator are you?

As the picture of the front cab mounts show, it is a simple design and could be copied. Check energy suspension for parts and inspiration.