Workshop items would be available in the relevant item menu. E.g. if it is a poster that is setup so it is placeable in the corridor, you can find it in by clicking the "items" button below the button you use to build a room. All workshop items will be indicated by a small blue/white square in the top right corner.

If an item can only be placed in a specific room, you can find it when you view details for that room and click its "item" button.