Hi.  If you are following one of the many recipes for paper mash, don't worry - there are replacements for ground chalk.  Generally it has been known as whiting and sold in decorators' shops.  Nowadays it is quite hard to obtain.  Polyfilla type products, even plaster can be used, acting as a filler, a smoother and a hardener.  The harder the plaster you use, the more difficult it can be to sand the final product.
If by any chance you are intending to use large quantities or are part of a group who might go shares, I know a source of a very high quality ground chalk but you have to buy in large (I think about 60kg) batches.  Ask me if you need to know more.
DavidO

ground chalk, whiting, gesso powder - all the same. I recently got some whiting at a ceramic supplier. Apparently it is used in clay. I paid $1.00 US for a pound.

Julie

I bought ground chalk at a home building store.  It comes in a plastic bottle and is normally used for refilling plumb bob gizmos.  I bought white, but next time I might try blue.  The surface I'm coating with the gesso/chalk mix is white (toilet paper papier mache mix) and sometimes it's hard to tell what you've already painted.  The mixture seems to work very well.