I can't really think of an instance where I'm looking for an item that isn't being asked for by an NPC or required for a recipe that I want to make and in both cases you can pull up the context menu on the item in question and hit search (or find item or whatever the wording is) which will search your own inventory and all of your retainers and tell you how many NQ and HQ you have, if any.

I suppose you weren't really asking for that, but I can' tthink of the situation where you need to find an item completely out of context.