Topic: Where to buy a blackboard made to attach to a cycle?

Strange question, I know, but there are a couple of bikes left locked-up around Winchester where I live, and each of them has a blackboard attached to it, advertising upcoming events at a local bar.

As I'm looking to start booking regular shows at a couple of local venues, it seems to be a good idea to buy a cheap bike (which I would also use as for cycling from time to time) and effectively join in with this reasonably effective mobile form of publicity.

Has anyone else seen anything like this before, and do you have any idea where I'd get the special blackboard from? It attaches to the back of the bike, and upon investigation looked as though it was attached, not an original fixture of the bike.

Cheers in advance for any help