There are 2 options for GH themed house, first is the Dragon Fjord in the crown shop, the other is Watchtower Hall which is crafted. There isn't a traditional castle vendor as in other main storyline worlds.
There is no official housing shopkeeper, but there are two options for Grizzleheim houses. The Dragon's Fjord available in the crown shop for 100,000 gold or 10,000 crowns or the Watchtower Hall a craftable house,is available from Torald Wayfinder, the recipe costs 65,000 gold and the ingredients are other crafted housing items. Personally I like the Dragon's Fjord for the Raven-like fortress and the ice.
There is no castle shop in GH or WT. GH does have a furniture shop, though--it's inside the gates of the fort area, and marked with a (kind of obscure) sign. If you want a GH or WT-themed castle you have 2 choices right now. You can craft the Watchtower Hall, which is really cool. It's my favorite house in the whole game. You have to be a Grandmaster Artisan to do it, however, and it takes a lot of crafting work--it took me about 6 weeks. Or, you can buy a Dragon's Fjord from the Crown Shop. It's in the GH/WT style--I can't remember how much it costs, though. Maybe 10,000 crowns?
Those are your only options for this style of housing at the moment.