Hunting for fanart of the 'your girlfriend was amazing' scene is one of my favorite little internet quests—it's like following breadcrumbs left by other fans. I'd start on Pixiv and DeviantArt because they have huge artist communities and reliable tagging. Try variations of the title, character names, and scene descriptors: use both English and possible Japanese translations or romanizations, plus tags like "kiss", "goodbye", "confession", or whatever emotionally defines that moment. Artists often tag by scene emotion rather than precise words.
If keyword searches come up short, take a clean screenshot of the frame you love and run it through SauceNAO, Google Images, or Yandex reverse-image search. That often pulls up reposts, artist pages, or booru entries (Gelbooru, Danbooru-style sites) where the image is cataloged. Don’t forget Tumblr and Twitter (X)—search hashtags like #'yourgirlfriendwasamazing' or #fanart and follow reblog chains. I also check dedicated subreddit communities for the series; folks there compile archives and sometimes have rare pieces. Above all, respect credits and support artists via Patreon or commissions when possible—nothing beats sending a "thank you" tip to the creator who captured your favorite moment. Feels great when you finally find that perfect piece.