You could try local (to avoid post cost) car paint and body shops.
There is also a list of metal suppliers on the forum somewhere. Generated by member. Note though some may be out of date.
I use One Stop Metals in Trafford Park. I only get small bits of sheet metal for car body repairs so they usually have an offcut or if not they will cut you a bit. Doesn't seem to be a minimum size or charge, and the guys in there are very helpful.
How about old computer case sides? these are about the size you need...ask people you know who might have old comps?
Or look on ebay for the cheapest case and ask the size,taking care to make sure thsides are all flat ie some cases had indentation for easily pulling and some have holes where air is drawn in from the internal fan.
Phone a couple of local smaller sheet metal firms and ask, don't ask the big ones as there is too much red tape. Hand them beer tokens and it shouldn't cost much at all