Just realized I forgot to post here
Last week I published a pack of 5 free 2048x1024 HDRIs here: http://hdrihaven.com/teaser.php
They were made specifically for one-click lighting in Cycles - and in fact I actually used Blender to help make these when commercial software couldn’t cut the mustard.
If the free 2k versions aren’t high-res enough for you, you can buy 16k versions for $0.79 each on the Blender Market.
Both the free and paid versions are CC-BY, meaning you’re free to use and modify them for any purpose, and even share them with friends if you like