What's the best place to get a single steel I-beam?
Yellow pages (or google, or...) and "Structural steel" (or structural steel suppliers, fabricators, erectors, etc.) Many may not really want to work with you (there's not really any classification of "suppliers of one beam for a house project" so some of the companies listed may ONLY play in the "build a skyscraper" market) but that's who you need for structural steel, and someone will supply residential one-offs, - ask for a recommendation if the company you call first can't help you directly.

Hiring the erection as well as fabrication and delivery is highly recommended. They know that they are doing and they have the right tools and equipment to do so safely. It's all too easy to kill yourself or your helpers when trying to creatively hoist heavy metal into place without experience and tooling for the job.