You can access Pokevision through the official website.
Tips to track Pokemon using Pokevision
Rarer Pokemon have a shorter appearance time You have to be zoomed in a certain amount on the map for it to work If the Pokemon GO server is down, then will Pokevision not work The map does not tell you where incense or lures are. Those Pokemon will not show up on Pokevision You can scan anywhere there is service, so not just in your immediate area
The bottom right tracking system on the Pokemon GO app is not working very well, so if you are trying to catch ’em all, this website will be very useful for you.
Check out Pokevision! (Or, you can also check out our guide to tracking Pokemon through Ingress maps.)