That is the mystery part of the game! :) Undiscovered items will not be available in the book until you explore them. Once discovered, the info text box above the board will guide you.

You can click the "i" (info) button to check the items' highest level and see whether the item you are looking for is in that chain. You also have an option to tap on the items on the order checklist. It can help you understand which chain you are looking for.

Chests are excellent sources for special items and also allow you to spawn new items. Focus on merging higher-level items if you feel like you cannot find the items in the orders.