My local IKEA (and others in Germany I've been to) has an inventory number on a tag of every item. Next to it is the shelf number and section within that shelf where I can find that particular item in the warehouse. IKEA also provides pencils and small pieces of paper with lists already printed on them to jot down any shelf + section + inventory numbers of stuff you want to grab.
Is it not like this in your local IKEAs?