With sizes like that I think you need to be looking at bulk suppliers rather than retail suppliers, e.g. hobby shops. Bulk suppliers like to deal in bulk, so they may well have a minimum charge irrespective of the actual cost of what you want to buy if you're only buying one sheet.
If you do go the retail route, then you're almost certainly going to have to buy smaller sheets and then glue them. Possibly, a retailer could get you the large size you want from their suppliers.