I get mine at Walmart. A little over $2.00 for a 20 x 30 size. I don't know what they cost if you buy in bulk (I don't do that many cakes, so I just buy one as needed).
They are in the office supplies dept usually.
And CakeIMake - I use them as my cake board. I take a foam core board, cut it in half, glue the halves together and then cover with a plastic table cloth (they come in a ton of different colors). They are very sturdy and inexpensive so they can just be tossed after the cake is eaten.