Go to Lowe's or Home Depot (lumber store) and locate the White Panelboards (in the paneling section of the store).
It is 4 ft by 8ft which cuts into 24 pieces of white board (12 inches by 16 inches).
Did you know that they will cut it for you? It is just a couple bucks extra and bam! You have 24 white boards ready to go for under $15. Can't beat it!
Just use kleenex or paper towels as easers!