MalibuDarby also has some really nice looking tilesets. Personally, I am a big fan of finalbossblues. There are plenty of independent artists that sell RPG Maker formatted tilesets that are able to be used in any game engine. Just be careful to not use any official RPG Maker tiles in your GameMaker Studio 2 games, as they are only licensed for use in RPG Maker. You can also Google "RPG Maker A2 tiles" to get ideas for the different kinds of things you can make with autotiles.
If you are interested in making your own A2 tilesets, I would recommend this YouTube series by CrackedRabbitGaming ( Part 1, Part 2, Part 3). The way it does this is by splitting up the tiles into "minitiles", which it then combines in various ways to form all of the necessary autotiles. It is basically a way to generate 47 tiles from only 5 tiles. If you are unfamiliar with this format, the article Anatomy of an Autotile explains it very well. This script expects the tileset to be formatted in the RPG Maker A2 tile format. Personally I use Shift+A because it is easy to press and isn't used by any other Aseprite commands. Then simply search for "GMS2", and assign the script to a shortcut of your choosing. I highly recommend assigning the script to a keyboard shortcut by going to Edit -> Keyboard Shortcuts.You must close your Aseprite application and open it up again in order to refresh the scripts list.Then simply drag the Lua file into the folder.
The easiest way to install the script is to open Aseprite and go to File -> Scripts -> Open Scripts Folder.Older versions of Aseprite might work, but I would recommend updating to the latest version available to you. This script was tested using the latest version of Aseprite at the current time, which is "Aseprite v1.2.21".I would recommend buying it from the website, as it gives more money to the developer and also comes with a Steam key. This script requires the pixel art application Aseprite.The script will copy whatever frame you currently have selected.
You can also use Aseprite's animation functionality to store tilesets in multiple frames. You can build your tilesets on multiple layers, and the script will copy any layers that are currently set to visible into the final generated autotile sheet. The script supports most of the features of Aseprite. It also includes a preview function that allows you to see how the tileset will look directly in Aseprite without having to import into GameMaker Studio 2. The script can generate both the 16 and 47 autotile formats. This is a script for Aseprite that will convert the RPG Maker "A2" style tilesets into an equivalent autotile sheet for GameMaker Studio 2.