Just change the range on the "select unit" ability in the object editor to allow purchase from any point on the map (rather than just "nearby" patrons). Or, if you have no units period, use a hidden, unselectable nearby patron (permanent invisibility, locust dummy unit). Wasn't sure which you were asking for due to the lack of information in your question, so just gave ya both answers.
An example of one of my maps that does it:
https://www.hiveworkshop.com/forums/maps-564/last-tower-v1-12-a-164641/index5.html