Most vinyl stockists supply chalkboard vinyl which can be applied. I found it works best for all the pub jobs that ive done and great for a-boards. The onyl down side was that it was difficult to wrap round, but the little heat does the job, though will have to be careful as it can detoriate through crackling. Other than that good piece of vinyl to have on the shelf. I know metamark and Allprint are the cheapeast, or ask your suppliers if they do it.
There is also dry-wipe vinyl too. And you can get special liquid pens that write straight onto the vinyls which could be a good selling point with the job in hand. I’ve now found that my local art shop supply these cheaper, unless someone knows even cheaper. Ive attached a pic of these.