If you’re in search of isometric assets that you can use to make maps in Tiled, I’ve found that many wikis of popular computer games post the assets in PNG format that you can freely download (for personal use only, of course).
In total, there are tens of thousands of high-quality assets, covering a variety of genres.
I made this YouTube Playlist that lets you easily preview the assets available from five different games. I hope some of you find it useful.
Be warned, though: downloading this many assets can be a real chore, especially since you need to manually trim the end off of each URL, in order to get the asset in full resolution, and not a downsampled thumbnail.