I think it's 10
different rare items, so you can't just farm ten Barettes unfortunately.
However, there are like 18 rare items from 18 different enemy types, and some are common drops (meaning some of the monsters have both rare and common carcasses you can use, and you'd need two of those monsters). Just have to get 10 of those 18.
Getting a pig and having it breed into the other two pig types will cover five of the ten needed items right there, because some pig types have a common and a rare that count. Hydras are the same way and could cover six. Unfortunately I missed getting the pig it gives you in Chapter 3 and now I can't seem to spawn them AT ALL in Chapter 4 (supposedly they spawn in Terminus, but no luck).
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This page lists all the monsters in Final Fantasy Tactics, how to poach them, and how to get Rare Wares.
www.rpgsite.net
Scroll down to the chart and just get some of the bolded items, cross them off once you have them. Pretty easy trophy overall. The only endgame trophy that is really any kind of hassle has turned out to be the "max all jobs on one character" because that's just hours and hours of grinding.