It's been hard for me to find a wide selection beyond those you mention. For the less common ones, FH Steinbart has Bramling Cross & Target; Yakima Valley sometimes has Target, Jester, First Gold (they have a Mystic & Harlequin too, that I know nothing about); Northern Brewer -- the site-- has Northdown, First Gold, a few others, but their prices can be high. MoreBeer can be hit or miss on less common hops. I could've sworn I saw Bramling-Cross on Farmhouse before, but not now. I've been wanting to try Sussex too, but I never see it.
I agree that it would be cool for us in the states to be able to get a wide range of UK hops all in one place.