Karl, Diamond State Recycling is where I go when I need specific scrap. I know the guys there and they are a good bunch, but you have to play by their rules. Explain why you are there and what you do. Wear long pants, gloves, and steel-toe boots. Bringing a hard-hat may be a good idea as well(if you don't own one, I can give you one as I have a dozen or so)... DON'T just walk into the yard, go inside first. A good time to go is during lunch break, as the cranes aren't running and it is safer to walk around and they are more likely to let you in. Where is your shop at? I am in the Wilmington area as well..
For new stock, Steel and Metal Liquidators in Newport is a good place.