The 'Undertale' fandom has some of the most creative fanart I've ever seen, especially when it comes to AUs like Nightmare Sans and Dream Sans. For high-quality pieces, I usually start browsing on DeviantArt—there's a treasure trove of artists who specialize in darker, more intricate designs for Nightmare Sans, while Dream Sans often gets softer, ethereal treatments. Tumblr’s also a goldmine if you dig deep enough; tags like '#dream sans' or '#nightmare sans au' pull up hidden gems from indie artists who don’t always get mainstream attention.
Another spot I swear by is Pinterest. It’s like a rabbit hole of interconnected art boards—once you find one piece, the algorithm keeps suggesting more. Twitter (or X) has a vibrant community too, especially during AU-themed art weeks. Just follow hashtags like '#undertaleau' or '#dreemurrfamily', and you’ll stumble upon threads where artists drop WIPs or full pieces. For something more curated, check out artists like Zarla or Sour-Appled on Tumblr—they’ve defined a lot of the AU’s visual lore. Honestly, half the fun is losing hours to retweets and reblogs, discovering artists who put wild spins on these skeletons.