@notjustbikes I'm also surprised you went out of your way to book BBBase!
Unless it's changed since 3 years ago, it must be purchased through JR's travel system. Only available online (in Japanese) or at staffed "green window" locations at major hubs.
You typically select a recommended bike route and are then given the BBBase tickets that match it.
The BBBase is further only available in late spring and early fall on weekends and some holidays.