I would need to see the comb to be sure, but looks like a possible Campine, Buttercup, or something else...
see I was thinking campine too but they are rare enough that most hatcheries don't carry them, but since you think so too then it very well may be, I don't know about buttercup I don't think there is enough speckling in the breast but same body shape so it is a possibility too.
Some feed stores will be assorted rare breeds. That's how I got my Campine, Buttercup, Lakenvelder, and Sumatra.