There's 3 at the airport, but you'll get 4 stars when you get near.
I found one yesterday on top of a Police station in Algonquin.
If you have the safe house Roman buys after you rescue him from the warehouse it's along that road to the north. It's probably easier to spot it in another helicopter and I'm not sure if it's always been there or if the story needs to be completed first. I don't have 100% completetion (can't be bothered)