As you progress through the ranks, you may find you are struggling to find specific resources and commodities you need to upgrade some of your buildings.

Higher level regions of the Empire Map will yield rarer resources and commodities. Be sure to check what the Towns and Villages in regions such as the Outer Empire and Frontier are producing.

However, not all resources and commodities can be found on the Empire Map. If you can't find a village or town producing what you are looking for, this is likely the case.
Instead, these commodities can be acquired through other means such as Importing from Capitals and voting for specific resources through the Tribute Hall in the Senate.

Alternatively, as your game world progresses to higher Epochs, new resources will become available in the general loot pool. These will then be available as rewards when defeating Barbarians. You can check what resources will be added in future Epochs by clicking on the Epoch flag at the top right and then opening the information panel.