Here are a few places I go to find art. Most notably https://openclipart.org/
The big trick with finding good stuff, is to obviously use good keywords, think outside the box sometimes, and edit the images a bit if need be.
Here's my list:
You can also search creative commons for images here: https://search.creativecommons.org/
Here's another good article regarding the subject: https://www.geeknative.com/49365/4-great-places-get-free-art-rpgs/
Keep in mind that in some cases, art is not always useable in a commercial project, Flickr is a good example. Take a look to see what version of creative commons things are licensed under.
Obviously sites like drivethrurpg.com have tons of great art, a LOT for sale. I've often periodically checked the stock art area and then revised my search by price. Here I made it easy for you, click on the link.