You must be on Staunton Island and on the mission "Search and Rescue" which is given to you by Sal over the pay phone. Go pick up the limo from Sal's garage, then drive the limo to YOUR garage. Do NOT go to the yellow marker. Go straight from the Sal's garage to yours. After the limo is in your garage, fail the mission (die or blow up the limo while it is in your garage) and then the Bulletproof Stretch is yours. This works with other mission vehicles as well.