It is inconceivable to me that Apple would allow any third party access to proprietary engineering information like a printed circuit board design AND allow that third party to sell into the repair market against Apple. Not to say that Apple doesn't allow third parties to produce boards for them (of course they do), but the only source for new boards is going to be through Apple (and that is even if they still support that model for repair) or other authorized repair depots that buy new logic boards from Apple and incorporate them into repairs. You will find other sellers of logic boards for iPads, but they are going to be used boards harvested out of broken display iPads, etc. Of course, you're welcome to look on alibaba and see if anybody is selling a new board out the back door of a Chinese factory, but that at that point you're basically dealing with a thief.
[doublepost=1511395416][/doublepost]Just for example, ifixit.com is a well-respected third-party source of parts and tools used to repair Apple products. Many of the parts that they sell like batteries and screws and adhesive tape are not sourced from Apple. I'm sure that their first preference would be to sell non-OEM (non-Apple) parts for every part that they can source; it would allow them to offer the part at a lower price, and likely have a better margin. However, when you go into their system and pull up an iPad Air 2 logic board that you want to buy it says "This is a used OEM part." So if ifixit can't get new, non-OEM logic boards it's pretty certain that nobody can.