pixiv tag: けいおん!スーツシリーズ (K-On! Suit Series) and スーツシリーズ番外編 (Suit Series Bangai-Hen, Extra Suit Series)
Artist: Ikari Manatsu
Characters, mostly from K-On!, in business suits, some in various states of undress.

• pixpedia article(JP): K-ON! Suit Series, Extra Suit Series
• List of Ikari Manatsu's series