Just look in the yellow pages. Don't go to the big suppliers, they have fixed prices and usually won't budge. The size you want will just come out of the scraps/discard bin anyway, so go to a smaller supplier that will be happy to get rid of a couple of offcuts. The price will be more if you need the edges polished - they usually charge by the metre for this. Sliding glass - make sure it's at least 5mm. Ring around.
Jamie