Update: there are now free pixel art UI kits too, if your game needs menus, bars or inventory slots:
- Pixel art UI kits (free Lite versions): Fantasy, Horror, Retro arcade, Cyberpunk neon and Cozy: panels, dialogue boxes, buttons, toggles, hearts, slots and bars, at native size and 4x.
- Pixel Cursors & Icons: 46 cursors and icons (hearts, coins, keys, weapons, UI symbols).
- Essential UI SFX: 150 menu and interface sounds, WAV + OGG.
Same deal: free for commercial use, no credit required.