Best Source for Purchasing The Strongest Rearguard LN?

Formally I was okay with buying all my books through Amazon because I could convert them with Calibre should I ever want to read them on a non-kindle device. But my understanding is that it is no longer possible do to changes implemented by Amazon. Just caught the first episode of the Strongest Rearguard and was thinking about purchasing the first volume. Would prefer Amazon still because of credits and gift cards, but considering another source in case I ever get a non-kindle device.

Thanks for your help. Hope you had a great weekend.