If you're being conscious about using only free-to-use assets, this page lists no-cost, free-to-use game assets.

final boss blues offers some (classic SNES RPG) graphics. There are also premium assets, so just stay alert as you explore the site.

Game-icons.net offers free (modern, two-tone) game icons. There's even a section for GUI icons (play/pause, download, refresh, wi-fi signal, etc.).

itch.io has (tons of different assets). This is how I usually find new things, so I can practice mashing random things together into a game. It's like drinking out of a firehose, with its infinite scrolling.