If you are new to cel collecting, and think you see a cell I would be interested in for sale or trade, confirm that it is not owned or sold by a cel flipper/scammer like TimelessCelGallery. The animation cel collectors reddit (r/AnimationCels) is a great resource for beginner collectors, sellers, and traders!
Generally, I have seen success with AnimeCels, ComicArtFans, Art-Toons, kimba4ever, and silvrdrago. I have never used Heritage Auctions, but I've heard it recommended by other cel collectors before.
Any cels of Acetylene Lamp, from any series, especially Black Jack or Jetter Mars.
Any cels of Hamegg, from any series, especially Kimba, Astro Boy, Jetter Mars, and Dororo.
Any cels of Sir Crocodile from One Piece. While it's wildly unlikely that I'll ever get my hands on any One Piece cel, on the very distant off chance that the opporitunity arises, I've listed it here.