I have several objectives that require the building of a structure that I don't have blueprints for (oil refinery, apartment block, etc.). The game indicates that such blueprints have a price, but how do I go about making the purchase?
Simply attempt to construct the building from the build menu!
When you click on a building which you do not have blueprints to (which are indicated by a special icon) a Penultimo dialogue box appears offering you the option to buy them.
In some missions, buildings are disabled for all or part of the scenario (with the line that Penultimo 'lost' them). The cost of blueprints are reduced by an Academy of Science set to 'Research Grants' work mode.