CMF,
I do not know of a web-site for Beam prices. Steel prices can, and sometimes do, fluctuate daily, albeit only a couple of cents, but it can happen. To maintain a website and get good pricing to use could be difficult.
I would suggest using a vendor that is closely located to where you are and begin to establish a relationship for your price and delivery needs. There are an abundance of steel suppliers through out the United States that can provide the prices and competitiveness you would want.
Hope this helps,
Scott