I got lucky, an FBI car just got lured into my Staunton safe house. I ran to collect the bribes as soon as I could, turned around, and then car was there for my taking
You get it as your second safehouse during your last mission for Salvatore. Maria pages you to meet her at the docks in Portland, then you take her to Staunton Island. After the cutscene that plays afterwards, you automatically get it.