Good news: strong Google-fu led me to find the specific wine that my grandmother was
adamant must be used to yield my great-grandmother's famous Christmas cookies, pastissettes. I have been on the search for this wine for about 8 years, and have never found it in any liquor store. I was starting to think I must have written it down incorrectly, or that my grandmother was somehow wrong. But I found it today online!
Bad news: ...they don't make the wine anymore.
ETA:
Good news: further google-fu reveals that there was a 2011 vintage of this wine! Yes! It DOES exist!
Bad news: ...but bottles run $15-$30. And apparently only exist in Germany, Spain, and Italy. Sigh.