Health & safety wise, new glazing has to be a certain thickness, quote the regs at him.
Also, if you should be lucky enough to find an ancient pane of glass you can cut to size...it's a nightmare to cut!
It's just so thin and brittle with age.
Or just get hold of some old greenhouse glass, there is plenty of that about, go visit a local glazier and tell him you need a piece of glass for your shed or greenhouse, they are sure to have some old offcuts lying around.
Got any local garden centres around you? Speak to the manager, or one of the guys out back, they'll have glass lying around too, and it'll be old ropey stuff....just the job

Ian