What Intel currently has available can be found here: https://click.intel.com/intel-nuc.html?limit=32 Intel® NUC Spare Parts. If what you want is not listed there (or is out of stock), well, you will just have to check back again later. As far as I know, there is no one you can contact that can provide information regarding what is (or is not) forthcoming.
Sorry, best I can do,
Gee, ask the right person and...voila!
https://www.intel.com/content/www/us/en/support/articles/000005873/mini-pcs/intel-nuc-kits.html Replaceable Covers Information for the Intel® NUC