List of most commonly recommended Tilesets around the sub. Please link any more you think look good in the comments!
https://eeveeexpo.com/resources/?prefix_id=9
https://www.deviantart.com/j-treecko252/gallery/79196109/pokemon-gen-3-tiles
https://www.deviantart.com/magiscarf/gallery/41257023/tiles-and-mockups
https://www.deviantart.com/ekat99/gallery/77971354/public-pokemon-tilesets
https://www.deviantart.com/anarlaurendil/gallery/67137394/tilesets-pokemon-sacred-phoenix
https://www.deviantart.com/aveontrainer/gallery/68320549/tileset
https://www.deviantart.com/spherical-ice/art/FRLG-styled-tiles-705658259
https://www.deviantart.com/skidmarc25/art/Full-Johto-Gen-3-Tileset-for-Pokemon-Essentials-961991026
https://www.deviantart.com/thedeadheroalistair/gallery/76140908/pokemon-gen-iii-stuff
https://www.deviantart.com/akizakura16/art/Hi-res-Interior-General-Tileset-588725678
https://www.deviantart.com/chaoticcherrycake/gallery/51253007/tilesets
https://www.deviantart.com/the-red-ex/gallery/all
https://www.deviantart.com/luizthedarkmaster/art/Pokemon-Gyms-Custom-Tileset-847367326
https://www.deviantart.com/lo8jd/gallery
https://www.deviantart.com/madrvst/gallery
https://www.deviantart.com/avalypuff/gallery
https://imgur.com/a/gen-3-desert-snow-tilesets-qSnVT2C
https://new.reddit.com/r/PokemonRMXP/comments/1bm9u2n/pretty_petal_flower_shop_tileset/