Official Pokemon Go Download Locations:
Android
https://play.google.com/store/apps/details?id=com.nianticlabs.pokemongo
To install on Android via the web browser - sign into Google Play Store on the top right corner. After you are signed in, click on the "Install Button", it should bring up a prompt saying it it downloading to your device.
Apple iOS
https://itunes.apple.com/au/app/pokemon-go/id1094591345?mt=8
To install on iOS iPhones or iPads via the web browser - click on the "view in iTunes" button on the left hand side under the game icon. Proceed to download and install from iTunes.
Hope that helps! Happy hunting.