It was a poster bundled with 'Monthly Bushiroad', that's the magazine where's the official manga is running in.
Yes, I made way too much research on this couple in order to find pictures of them...
Me too... and I searched more than I want to admit for scans. I was even willing to buy Bushiroad because they have one or two more interesting series (BanG Dream for example) but they're print only as far as I can see. Inclusive shipping it's a bit much then... Digital would be a different story.