Hans, a former member of the Forum, Gwen Black, had a very large collection of Ranunculas ficaria cultivars but she moved house last year and we have heard nothing from her in a long time.
Perhaps Gwen is out there, reading this ? We'll see. I must try to contact her, last time the email failed.
Mark Smyth is very fond of these buttercups, too, I expect he will have some advice for you to source more types.