Somethingproof Vehicles
These are vehicles that are bulletproof, fireproof, damageproof, a combination of these... and some are even everythingproof.
Everythingproof Stretch
The Everythingproof Stretch is only everythingproof when it's in the garage; it seems that the designers didn't want anything damaging the vehicle before it came into use, so they made it impervious to all damage until after the cutscene where it backs out of the garage. It's one of the more annoying vehicles to get, but it's more robust than a tank. Absolutely nothing in the GTA world can destroy it save flipping it over or the "blow up cars" cheat.
Begin Salvatore's Called A Meeting and drive to Joey's Garage. Now there are a few methods, and not all will work on all platforms. Keep trying until you get it.
Walk Though Walls method
DO NOT park in the blue corona.
Take a vehicle such as a Belly-Up Fish Van and park it on a diagonal with the wall. Then enter the "speed up time" cheat twice and run into the wedge space between the two. With luck you should be forced through the door and into the garage.
Now you have it, BACK UP INTO THE WALL. The car will go through it and into the famous Blue Hell. After falling for a while and losing some health, the car will relocate outside the garage. Weird. Now, on to Staunton...
Kill Joey method
This may or may not work. Go in as normal and pick up the Stretch and Joey. Then continue as follows...
What To Do Next
After you've acquired the car through whichever method, use the Cars Can Fly cheat (Liberty City players: it's better to use SEAWAYS instead) and fly it over to Staunton Island. Don't worry, you can still get 100% completion.
Visit 8-Ball's bomb shop, rig it up, and then put it in the Staunton Island garage and blow it up without the door closing. Then allow the door to close and when it opens it will be repaired.
If Joey was in it, you fail the mission; if he wasn't, it is to be assumed that the game will supply you with another limo. In theory you could probably repeat this for a FLEET of Everythingproof Stretches
Some users have reported that when this is done it isn't everythingproof, so this might not work for everyone.
Other Notes
Whatever you do, don't take it to a Pay 'n' Spray. Just like the Asukamobile in GTA Advance this is a unique colour; if you paint it, it will lose its jet-black tone and you can never restore it.
Bulletproof Patriot
WARNING: mild mission-reward spoilers follow
Definitely the easiest to get. Just complete the mission Marked Man (Ray) and it is part of your reward at the end. Be sure to stow this in a garage, it will be VERY handy later in the game.