Carley Lamp can make them for you, up to 6" in diameter, but it will take time and they have a $50 minimum order. They make really excellent reflectors, though. They come in 7 different undercoating options: smooth, light orange peel, medium orange peel, heavy orange peel, light stippling, medium stippling, and heavy stippling, and can be vacuum metalized for an extra shinny surface. I recommend this option highly as it is very cheap and makes for a very shiny and easy maintenance reflector.
My 3" reflector cost $13 undercoated and vacuum metalized (also called "aluminized"). I suspect costs would rise rapidly for 4 and 5 inch reflectors. Call them and ask. They're a great company to deal with. I'm sure there are also many other options. Also, I suggest the medium orange peel option.