A repository with free resources ready to be used with the Tiled map editor.
There are many resources available online.
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OpenGameArt.org has a large collection of free assets, including many tagged tilesets. Also check out Thorbjørn's collection of tilesets with terrain transitions that should work well with the Terrain Brush.
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itch.io lists many free and non-free game assets, including many tagged tilesets. Thorbjørn is maintaining a collection of tilesets there as well.