DeviantArt is one of the most trusted and reliable stock image sites. Most WP artists get their stocks from there, as its easy to credit and find. Flickr is also a popular stock image source from what i've heard.
However 'free' stock sites and 'pay-to-use' stock sites such as shutter-stock, Pixelbay, Pexal, Dreamstime, Unsplash e.c.t. aren't permitted as they aren't reliable sites and often create 'stocks' with stolen artwork.
Other websites such as Dragoart and GetDrawings are also not allowed, due to the high amount of stolen artwork and theft.
Here are some links to guides and information sources if you want more info/detail about what stock sites are, and aren't allowed.
https://wolfplaygame.com/forums.php?f=4&t=100
https://wolfplaygame.com/forums.php?f=4&t=139
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