On the inside of the door to Ona's equipment locker the Directors have kindly pinned a list of all of the items in sleuthville.
To make it easier to find what items to upgrade to, they have put up seperate lists of hats, coats etc.
Do your cases, embark on a Villian hunt, master the favours, befriend the factions, or simply buy from the detective store. Prices are high in the stores. The friendlier you are with a faction the cheaper the cost.