Art is a major bottleneck for me. Game concepts, playtesting . . . all of that I can handle. When it comes time to make an attractive prototype, however, things grind to a halt. Nice artwork is beyond my skills.
I’m sure others have run into this issue, so I thought I’d ask: when you need art for a prototype, where do you go?
The Law of Game Design 
http://game-icons.net/ is pretty useful for a variety of things. I also hear people reference https://thenounproject.com/. I sometimes draw stuff myself. But usually I spend way, way, way too much time looking for _just_ the right image on Google image search, and wishing that I’d spent that time on the game mechanics instead.