Pocketbook does not ship to the US. (I wrote to them to ask.) If you can get them from Amazon, it's through third-party sellers.
I bought an Inkbook Prime a few days ago, and wrote an
initial-thoughts review of it. It's Android-based and has physical buttons; can't speak to Calibre collections support. (I can barely operate Calibre, and my Calibre library is cluttered by multiple versions of the same books, which have slightly different requirements to be submitted to different online distribution sites. I've never taken the time to use it as an organizing tool instead of a converter.)