I think the title of this blog pretty much sums it up. You need these in your classroom - more than you need textbooks. If your school won't buy them, get a class set yourself. They are about 9 x 12" in size. They can be easily procured by any online school catalog such as WB Mason. They come with hard backs or floppy backs. I like the hard backed ones because they are easier to hold up and just more satisfying to use. For a while, Dick Blick had the best prices but it's been a while since I checked.
You also need whiteboard markers. Now these, your school should definitely supply. If your school doles them out three or four at a time, be adamant that you need the entire box. Make your case that you are not a hoarder but you will need 20 students to be writing at once. Smile.
You don't really need erasers. I tell students to use scrap paper to erase. I don't let them use tissues because tissues are also in short supply and one whiteboard activity can deplete your supply for the term. Other teachers have used rags which they hand out. I find these get nasty pretty quickly but they do erase the boards better than scrap paper.
You can of course share boards with another teacher. This will work to get you started. However, I use mine so frequently that I fear tension may develop over usage of the boards. Sharing is not my forte. Regardless of how or where, get a class set like the blog title says.