Yosef Karo, Translation:Shulchan Aruch — 656 One must pursue beautification of the mitzvah when buying an etrog
“ Shulchan Aruch Orach Chaim 656 One must pursue beautification of the mitzvah when buying an etrog שצריך לחזור אחד הידור מצוה בקנית האתרוג 656:1 If one bought an etrog that can just barely fulfill the obligation, i.e. if it was exactly an egg-size, and afterward found a bigger one, it is a mitzvah to pay a third more than the price of the first to swap it out for the nicer one. There is one who says that if there are two etrogs available for purchase, one nicer than the other, the nicer one should be purchased if it is not more than a third more expensive than the other one. ”
