Icons8 is one that I use. You can use them for free if you link back to their site as credit somewhere on the home of your website or you can pay a fee.
There is also FontAwesome but those are more basic icons as opposed to icons that stand out and look great.
There are lots of sites where you can download free icons from. Go to Google and search PNG downloads, I am pretty sure lots of sites would pop up. Or you can simply visit sites like pngtree, or pngwings. I make use of pngwings, and it has been delivered so far.
