For old-school pixel art, try looking through the sprite sheet assets of old 8-bit NES games on The Spriters Resource.

For contemporary pixel art, Yacht Club Games, makers of Shovel Knight and Mina the Hollower, is an absolute gold standard.

https://www.spriters-resource.com/nes/mm2

https://www.yachtclubgames.com/blog/breaking-the-nes