You can also check out any local junk yards -sometimes they turn up there.
Yes they are thick enough, but each sign is different. I have two shields made from street signs. One is perfect, while the other one could be better...
if you want somthing other then street signs you can purchas it through any metal supply. if you are up to the work you can cast your own sheets from scrap aluminum obtained form the local scrap yard.
I checked with my local highway department. I told them what I was looking for and they let me raid their scrap bin. They told me I could take any of the signs that were not perfectly straight (Silly them). I left with about 50 signs ranging in size for 12" by 12" to 4' by 8 '. They were happy too since they didn't have to pay any recycle fees.
To expand on what Hjalmr said, look for the signs that come off of highways for the harder al. The normal "city" signs are usually softer but make good armour. The highway stuff is better for shields.
I work for a municipality and can get all the retired, dog eared and bent signs I want. All you have to do is ask your local public works (great oxymoron) or your local D.O.T. if you can have their leftovers. They usually will ask you to not dispay or paint over the informational side of the sign. You will get asked where you got the sign from if you don't paint over it.
