There are a couple of options for you,
Hand Nibbler; this is a hand held/driven device worth about $30. Will on with most panels on your car needs a 12mm hole drilled to start. Good excersise for your forearms.
Power Nibbler, sorta like a jig saw to look at will do most thicknesses of panel needs a slightly bigger hole to start . Much easier on the forearms.
Power Jigsaw, about $50; will cut flat panels fairly easily as long as you use a metal cutting blade. Don't use the wood cutting blade makes a serious mess of everything.
Power Saber saw; Uses half a normal hacksaw blade, will cut everthing your hearts desire if you have enough time.
Air Hacksaw; If you have access to an aircompressor, this is a magnificent tool. Much better than an Oxy for surgically cutting up car bodies. It also uses half a normal hacksaw blade.
Hope this helps, these tools are available at real tool shops.