BTW, even if you're not interested in playing Ingress, I'd recommend for Pokemon Go players to download it if only for access to the Intel map, since there doesn't seem to be an equivalent yet besides some crowd-sourcing.
Not every Ingress portal is a Pokestop, and it won't help you find gyms, but if you're in an area where things are more sparse it's probably helpful to find hotspots. (If you're in a big city, you probably don't need this, and also I hate you... I miss my couch portals!)
Install Ingress, make an account, and you should be able to log in to the Intel map at
https://www.ingress.com/intel -- in my area at least 75% of the portals on the map seem to be Pokestops or gyms, but it probably varies. Not sure how they decided what to copy over, but from what I've explored so far dense areas are dense in both games. You'll need to zoom in some to see every portal, because low level ones are hidden by default.
(I'm also seeing now that most of the area I was playing Pokemon Go in today is full of UNCLAIMED Ingress portals that I don't even have keys to, and I'm cursing my decision not to bother with it today... But I wasn't alone, and I didn't need to be staring at my phone.)