I found a link to Amazon Japan for さよならの朝に約束の花をかざろう (the Japanese title for Maquia: When the Promised Flower Blooms) manga, but I can't find the English edition/translation.
Where can I buy the English edition?
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Unfortunally you're out of luck. according to MangaUpdates it has yet to be licensed for English
However it's currently only 1 volume in Japan and on-going having only came out last year so it's a new-ish series. so it's still possible it could come out