Just like with the photos, there are many websites where you can find quality illustrations and vectors (usually SVG format), under public domain or Creative Commons Zero license, free for commercial use.
Finding CC0 licensed illustrations and vectors is much harder than when photos and number of websites you can use for that is rather small.
If you need free photos for commercial use, check out the previous article: 7 websites to get free images for commercial use.
CC0 is the only Creative Commons license compatible with GPL license. So, here are 7 websites with high-quality vectors and illustration, all under Public Domain or CC0 license, and free for commercial use, with no attribution required.