I'm guessing this is a popular item since I don't see it being sold. It's the last thing I need to start building a big building. Any advice on where to farm it? Urns, pick-pocketing or both?
You'll need to farm it in Summerset. The monastery in Shimmerene is an easily accessible farming spot.
Another source are the blue Alinor furnishing documents from Faustina Curio, which require Summerset to purchase. These documents cost 20 vouchers and contain a random blue Alinor furnishing plan.
You know, now that you mention it, I don’t think I’ve seen that furnishing plan in guild traders for some time. That is odd. I’ve sold a few of them in the past.