You can try Adorama and B&H (links below), but I don't think it's likely they will have one.
There's also KEH (who I think are in Atlanta).
The problem is that it's a very popular lens, so if there is a used one for sale anywhere it will sell pretty fast. It also holds its value well, so you may not end up saving much and buying a new one (with warranty) might be a better bet.
If all else fails, you could look at the
EF 70-200/4L, it's $400 or more cheaper and very sharp.