IngramSpark. That's the end answer. Ingram essentially has a monopoly on the wide distribution scene. Even though there are other printers who also promise wide distribution, they use Ingram's distribution channels to get the books where they need to go. (Yes, even Amazon uses them for their distribution channels into bookstores). Many bookstores won't even look at stocking you, if they can't buy it from Ingram's book catalogue. You might as well go direct with IS, to cut out middleman fees.