Definitely it is quite hard get outside of Japan and also hard to get outside of Greater Tokyo metropolitan area. The sales in Tokyo, Kanagawa and Saitama prefectures exceed 80% of the total sales in Japan.
Hoppy is a beverage produced and sold by Hoppy beverage and it is a beer-flavored drink which has 0.8 percent alcohol. Although it is not considered as alcohol by Japanese law, no one drinks Hoppy without mixing shochu with it.
Therefore if you go to izakaya tavern in Tokyo, you will have to order Hoppy set; which comes with a bottle of Hoppy and a beer glass filled with ice and shochu. Usually the bottle of Hoppy is for 2-3 glasses, you do not need to order another Hoppy set if you want just a refill of ice and shochu.
Usually ice and shochu; the refill is called NAKA. Therefore you have to ask the waiter as "Naka, please". Hoppy was invented a few years after the end of WWII, when the price of beer was high. Hoppy set is still cheaper than ordering a beer but people now order Hoppy due to the taste rather than the price. By the way, there are two types of Hoppy, the regular one which is called SHIRO(white) and KURO(Black). Kuro has a taste of dark beer.