They're everywhere!
In rural Hokkaido, some people even have them outside their home's driveway for people walking by. They have various teas (green, hojicha, jasmine, etc.), Coke and Pepsi products, Pocari Sweat (like Gatorade), iced coffees, and sometimes even hot teas and hot coffees that are heated on demand. They're super convenient and something I miss having in America (we seemed to have more of them here in the 90's and early 2000's).
The only problem is that in Japan there can sometimes be absolutely zero public garbage (or, more correctly, recycling) bins in sight. You have to carry your trash with you, which is a bit annoying and mildly gross if it spills.