EDIT: I have updated this tileset, and the updated version has been uploaded as well :)

Here is a tileset I have uploaded to OpenGameArt.org
If you want to use it for a game or project here is the link: https://opengameart.org/content/2d-dungeon-platformer-tileset
If you want to see how I made it, here are some videos:
https://www.youtube.com/watch?v=obSJ63a-0Bw
https://www.youtube.com/watch?v=iUnIkYAPe-8