I have a warped 5" tongue and groove board under my toilet, which is being replaced. The rest of the floor is in excellent shape. Any advice on how I might buy/find just one 5" t&g floorboard so I can get the floor looking as good as the new toilet?

What species of wood is it? How thick is it?

It's white oak. Haven't pulled it up yet, but it would be from a pretty typical 1991 random width pack?

You may need more than one board if the floor has been wet long enough to warp.
Look in the Yellow pages under hardwood flooring distributors or floor laying and refinishing contractors. They can tell you who in your area stocks the wood that you need.

Thanks for the advice!