Staples sells em.. They're pricey!  I worked there a couple summers ago, guy came in that was working on a movie set (Species 2, I think.. some B movie) an needed one in a hurry.. Pronto!  We happened to have one in stock, so I pulled it out, took it to the cashier for him, and loaded into his truck.  Gave me a $20 tip for my help (who was I to argue, I made $6/hour).
You're prolly lookin in the $300-$400 range for something that big... Most of the office supply chains should have em. 
edit:  By whiteboard, I'm assuming you mean a dry erase board.. If not, ignore my comments.....