My grandfather was a beekeeper in Montana and swore by spotted napweed honey. Of course spotted napweed is invasive and should be ripped out when found outside of its native Europe.
buckwheat honey could be an acquired taste for some. i love it. it's definitely available.
buckwheat is a summer cover crop. that means the (good) beekeepers would allow the bees to keep some for their own food for fall/winter feeding. also...dont know a lot of people who summer cover crop anymore. i guess its definitely niche, but certainly available in the usa.
i cant control where my bees go foraging. so i just label it 'wild flower' honey. in early spring before they swarm, i make sure the brambles are flowering. at that time..even in california..there isnt a whole lot of forage for them when they come out hungry. they also gorge themselves before swarming and so usually there isnt a whole lot early spring honey. its a timing thing. so really..there is no good time window for collecting buckwheat honey. unless we steal it from them and replace it with something else for their feeding.