"Trinkets" are the last section of your inventory on the right sight under Papyri and Animal Goods (you may have to scroll the section down to see all of them). Generally you find trinkets by looting small containers e.g. sacks/vases/boxes. Use Eagle Pulse whenever you're in a fort/villa/ship/hideout/whatever to help pinpoint containers to loot and grab everything.
For this trophy you don't have to have 100 of one trinket type, just 100 or more in total. Once you've got enough, go into a town and find a merchant (Weaver/Blacksmith/Stables), enter the Sell screen, and hold the
to "Sell unnecessary items", which sells off all trinkets and animal goods. (Side note: some animal goods are necessary for various quests.)