ArtMetal

The idea of Socrates Sculpture park is a good one, but basically, you are going to have a tough time finding any.

Cor-Ten is made by only one company- US Steel. And they only make it in a limited amount of shapes and thicknesses.
Its is very rarely stocked- only a few suppliers in the whole country.
It is expensive, and seldom used except when some wacky architect gets the idea that its cool.

So scrap is virtually nonexistant.

To rust evenly, it must be sandblasted after fabrication, and then it will drip rust all over whatever is below it. If graffittied or scratched, it will need to be resandblasted and rusted again- no touchup is available.

There is one company you might try, which seems to stock more of it than most anybody else, nationwide- and they might have some cutoffs or drops you could buy from them.

http://www.centralsteelservice.com/products.shtml

Cor-Ten is one of those things that sounds good on paper- but in reality, for most stuff, ordinary mild steel will work just fine. If sandblasted, mild steel will rust, and then can be clearcoated. A clearcoat will last a couple of years. If you really need a sculpture to last a hundred years, bronze or stainlesss is going to be a better choice, at 5 to 10 times the cost, of course.