There are many commercially available whiteboards on the market. You can simply google "whiteboards for education" and return thousands of results in various sizes and price ranges.
The whiteboards my colleagues and I use in our classrooms are made from shower board we purchased at a local home improvement store. The board comes in large sheets and we asked they be cut down to 24in by 36in. We can usually get 6 boards from one sheet. With good care, these can last several years. In time, the boards will begin to show wear - scratches will appear on the surface and they will become more difficult to clean. When this happens you may decide you want to replace your whiteboards.
To clean my boards I use Fantastik All Purpose Cleaner. This removes the marker residue. I then finish by polishing the boards with Endust. This final step makes it easy to erase the markers and allows me to go several weeks between cleanings with the Fantastik. I have found these specific products to work the best.