Can anyone tell me were I can buy the novel and is it available in English

Nah
https://m.facebook.com/notes/ero-blpages/novela-querido-benjam%C3%ADn-vol-1/625370901359254/

Think the English version just started getting translated, so right now it's just the first few ch's: https://18.foxaholic.com/novel/dear-benjamin/