First spawn villagers until you get a librarian (white robes).

Next, trade with him many times. You'll notice that there can be more than one slot for an enchanted book trade, and if you trade long enough I'd assume you would eventually get a separate slot for every single enchanted book.

This happened because NBT tags are now taken into account in villager trades, so if a villager detects that, say, a Silk Touch book trade has already been encountered, and then it attempts to create a trade for a Fire Aspect book, it will think that the offers are different, so it will create a new trade slot.

I haven't tested it, but if my explanation is correct this will probably also work for blacksmiths (who now sell enchanted diamond gear, on which the specific enchantments can vary) and priests, who also sell enchanted gear.