I'm not sure how folks feel about offsite recommendations here, but for plants I've enjoyed working with some of Ayene-chan's pixel art. She clearly states that her resources are available for free use with credit, even commercially (https://ayene-chan.deviantart.com/). Please note that she posts both fully original and derived works, make sure to stick with the fully original works such as the following:
I'm not sure how folks feel about offsite recommendations here, but for plants I've enjoyed working with some of Ayene-chan's pixel art. She clearly states that her resources are available for free use with credit, even commercially (https://ayene-chan.deviantart.com/). Please note that she posts both fully original and derived works, make sure to stick with the fully original works such as the following:
https://ayene-chan.deviantart.com/art/Nature-Tile-II-RPG-Maker-256940806
https://ayene-chan.deviantart.com/art/RPG-Maker-Trees-257683855