A new St John hospital traffic sign found under Decor / Misc. Decor, costs 0. You need Ambitions and to place the package in both the mods/packages (Sims3 ) directory and in the "The Sims 3 Create a World Tool/ Packages" directory for the item to show in your game. There is an earlier post explaining how to install items in CAW.
Importing packages into CAW
Alternatively you can place a small lot on the side of the road and place the sign on it.
St John hospital traffic sign (MediaFire)
St John hospital traffic sign 2 for CAW
St. John hospital