Anything not stocked can be ordered, so yes. I'm not sure about Lowes, but I have a family member in the major renovation business, and they swear up and down by one particular Home Depot that has come through in a pinch on multiple occasions. So I take they can get whatever you need fast, too.
Also, you don't necessarily need steel beams to build a house. You can make your headers out of wood. Hell, many of them are, you can nail two 2x12's together.
I'm not saying you SHOULD use HD/Lowes as your exclusive supplier to build a house, but it seems to be possible. And aren't absurd possibilities what we're all here for anyway?