You can buy in Bastok at the vendor, in Bastok Markets, near the spell shope, just to the West. 47 gil per.. .Sell about 1-1.5k Unicorn. ^_^ Good money make.
In all honesty, you wouldn't obtain these from the Virgin Yagudos. Let's face it, you can take it from them, but can't really obtain it. LOL just had to comment on that one.
these can be bought at AH, or bought at NPC. I bought one for DRK AF1 and it cost 44 gil at vendor. Yes they are stackable. If you have to do this quest: check region map for who controls Kolshushu. See the corresponding NPC from the controlling town. Only one Yag cherry is needed to complete the quest for all of the DRKs if you go with more than one
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"TF_MMO_Quest1" "You must kill %s1 %s2 to complete this quest."
From what i have seen they can stack, i have only 1 in my inv. so i can't be 100% sure, but it has a number 1 next to it just like crystals and such, that do stack.