Those look decent, but they have a grand total of eleven two-dollar EXRs... but hey, Apple started in a garage... Here are the ones I use most:
https://hdrihaven.com/
Great HDRs for $5.95 each (I think), and a handful of free full-res 16k.. but apparently closed until 10/23/17 as there is a countdown clock to that date.
http://hdri-skies.com/
Not as high-quality as haven, but a huge selection and all HDRs are free up to 2k, which is plenty for lower-res and those that don't need backplate.
great