Use any board that is lying around or buy some mdf and paint it with blackboard paint.
You can also make your own in any colour you like with a mixture of emulsion and tile grout, might fit in better for a child's bedroom wall.
This may not be of use because its free-standing. But I bought one with a whiteboard in the other side and also has a place to hang a roll of paper on it. Got the whole lot including markers, chalk and paper in IKEA.
i have bought blackboard paint in B & Q and painted it directly onto an area of the wall in the playroom. problem solved. hours of fun had by children since
When my kids were small I painted their bedroom doors with blackboard paint and the whole door become a blackboard, it was very popular and stayed that way into their teens when they used it for grafitti.