How do you find the stationing plans for the upgrade of the Town Hall?

Stationing plans are bought with gems. Although they are occasionally offered in exchange for resources in the Trading House -> Trade option.

To buy with gems you can find them in the Item store under the 'Other' tab. Or more simply just tap on the Town Hall and select Upgrade. If you don't have enough stationing plans then there will be a 'Get More' button next to them. Tap this and you will have the option to buy them at different levels. (it's slightly cheaper to buy a lot in one go)