OP wants to avoid Amazon. This is still giving Amazon money and therefore encouraging Amazon's greed, walled garden and dark patterns.
A better alternative would be using almost any other digital book seller. There's plenty. And if the author chose or was forced to only sell through Amazon, then you can try library sources (like Overdrive) so the author at least gets something. And if you still can't get what you're looking for, then it's time to sail the high seas.