Look around at any local fetes/markets, there is always a bag stall of some sort (with library bags, drawstring bags etc), if they don't have any that are the right side then I'm sure the stall holder will whip one up for you on request. It might even give them an idea for a new line!

Alternatively you could get a nice coloured pillowcase, and turn the top edge over and handstitch it down, thread a cord through...would that be big enough or do you want one bigger?

Also, places like Big W often have fabric santa sacks with a picture already on it, you could get one of those and decorate it further with applique, fabric paints, buttons, whatever you like.