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According to the article "Who Are Saved?" on catholicmagazine.news (near the bottom of the page), the aforesaid sermon can be found in:
"Prediche quaresmali". Assisi: Ottavio Sgariglia, 1806, v. III, pp. 146-182.
Some of the other Volumes of this work can be found on Internet Archive, but I was not able to locate Volume III there. Nevertheless, if one goes to Google Books the proof can indeed be found on p. 146, where begins---
"Del Poco Numero Degli Eletti" (Of the Little Number of Elect).