That is not an answer to the question on distinct levels:
1) All the items exist, we know they exist because the DMG says they exist, so we don't need to find out how they exist, even if only level 12 casters exist, maybe every single item in the game was wished up by Efferti. The issue is "How do PCs know the items exist in order to look for and buy a specific one?" The DMG explains over and over that the PCs get to come to their decision about what items to look for, and then they find them. How, in your twisted imagination in which you need a rules cite to know an item exists, do they do that?
2) How did the person who crafted the item know the item existed to be crafted? Do you have a page citation for your completely baseless claim that being able to craft an item immediately grants you knowledge of it's existence as a thing you could craft by psychic insertion?
And wishing for items that you know exist is something that we don't need gm fiat for.
And knowing that items exist is so basic, that if your DM declares you can't know about them, you can't buy them.