Kenney really do make some awesome free and paid-for art to use in games and they just updated their Kenney Game Assets 3 with a lovely set of pixel art interfaces.
It was already a big pack, but now even more so as it includes 230+ tiles for UI, dialogues, text balloons and so on. That's in addition to the racing kit, castle kit, space kit, furniture kit and many more packs included in Kenney Game Assets 3.
This is the same developer who made Asset Forge, a tool that allows you to make 3D models and 2D sprites by snapping blocks together and it supports Linux as well. The tool is due for a major upgrade soon which will be free for existing owners.
I'm a big fan of the work from Kenney, as they also give out a ton of free assets which you can find on their official site.