I managed to find out the source of a certain pic I asked the source of in the last thread.
Yesterday arrived the K-ON! Movie Official Guidebook (and also the second season album with the cassette) to my home, and it has this art in the very last page. Maybe this is the reason I couldn't find it in any booru? Anyway,
I also could not find any scan of this book on Google, so maybe I'll scan it and upload it, but I don't know enough moonrunes to translate the texts properly yet. Maybe some kind anon could, maybe not.
By the way, this book includes a random postcard featuring one of the five main girls, and I was lucky for once, so now I can proudly proclaim that I own even more Ricchan's merch.