Bikkuri Donkey and hamburger

Last night, we ate at Bikkuri Donkey, which is a"hamburger" restaurant chain. When you say hamburger in Japan, you mean chopped steak. The first time I went to a hamburger restaurant in Japan, I was surprised at the complete absence of buns - not on the menu.
There are plenty of fast food restaurants which serve hamburgers as I grew up to know them and those are also called hamburgers.
Last week, I tried to find some hamburger buns at the grocery store and couldn't find any. They are available, but you might have to visit a few supermarkets to find them.