Yes, you can buy ceiling tiles individually, Lowe's, Home Depot, or just about any hardware/lumber store should have them. But the problem with using a ceiling tile is that it takes the shape of the surface that the tile sits on. If you put it on a surface that is warped, then you will build that warp into your plane. A hollow core door is an excellent surface to build on because they are usually flat and free from warps, but if you're worried about putting holes in the hollow core door you can combine the two. Put the door on some saw horses (or some other surface to support the door) and then put down ceiling tile on top of that. The tiles will then be warp free and you can push pins into the tile.