They are aquired from the seven coloured scroll. as you use it more the resource drops become more common as you cant get rolls for the unique things like buildings twice and each of those is then replaced with a chance to get the chest.
I will likely implement a way to craft them with the next update (which is in the works!) like all the other resources. in the meantime, if you want you can go into the buildings_production. xml file in my mod and enable the debug option to craft them temporarily by un-commenting them.

that update will also have a fix (hopefully) for a few other things, although probably not the weird merchant pricing.

Found the solution of my problem. :)

Hopefully the update comes soon.