I would ask in the Trading Post here, and also ask Mike Richardson. You can find him at mc68000.org. I bet he has a couple dead 6100s that he could pull from. From my experiences with him, he is very reasonable with reasonable people.

Also check eBay, they look to be selling for about $10. Make sure to do your research so you know exactly what the SIMM looks like. People will often times misidentify these things. Here were my search terms:

(ROM, Cache) (Apple, Mac*) -cd -"cd-rom" -dvd -"dvd-rom"

Then just look through and find the correct ROM SIMM(s) by looking only at the pictures.