go to any local heating and AC company or sheet metal company and tell them you need some 18 or 20 gauge sheet metal, normally they will sell you as much or little as you like. This will weld nice and make good patches. Most of the panels on the car are 20 gauge, but you might opt for 18 as it is slightly thicker and less prone to burn through.
The allure of the aluminized plate is reduced corrosion but once you start welding on it, much of that burns off not to mention its a pain to weld through; generally you need to sand/grind off the area where you are going to weld.
Find someone place local, you never know when you might need more for a project and its good to have a source. Since you are making patches, they may have some scrap they will give you or charge you very little for. Know that metal is sold by weight so they might just throw what you want on a scale.
Last point, be cautious that you don't get galvanized metal or you will have to grind off the edges you will weld and welding galvanized produces a toxic gas that is harmful to breathe. That said, I will admit I have welded a lot of galvanized sheet metal, just make sure you have good ventilation and wear a full welding helmet. On cars, I definitely prefer clean sheet metal for patches on cars.
