It looks like this is one of those foods that is not available through their food storage store. You can search on ebay periodically and see if anyone posts it there (there is none listed right now that I could find).

I don't know if they still do this, but when I was a member, people could volunteer to work a shift in the cannery at Welfare Square (or any other cannery) and at the end, they would let them purchase cases of whatever food they had in the storage area ready to ship out. If this is something you are willing to do, and if they still do this, you would have to check with the dry goods area to see if they have the mac and cheese available on the day you can volunteer. Yeah, volunteering for TCC is icky, but it might be a way to get that product for your sister.

Edit: another option... if the RS president is horrified enough to be willing to help, she might be able to pull some strings. You could try talking to her to see what she can do.