Where To Buy Ouija Boards
The easiest way to buy a Ouija board is to head down to your local Toy's R Us. They've got Hasbro's glow in the dark board. You can also order them directly from their online store. While adequate, you may be looking for something a bit less "gamey" for lack of a better word.
You can always find Ouija boards on Ebay. A quick look just now shows that 282 Ouija boards are listed for sale. Everything from a vintage 1972 board to custom carved boards costing upwards of $100 each are listed. Include the terms "talking boards" and "witch boards" when searching.
You can also do a Google search for custom boards. There are quite a few sites that make amazingly detailed custom witch boards.