A good example of these are signs one sees on street corners or in parking lots.  They are aluminum and can be printed on one side or both.  These make great stall signs for mules when at a show or on their door in the barn.  They are very durable.  Not as many options in the size of these signs: 12"x18" ($45); 18"x24" ($54); 24"x24" ($81).  These signs are great for farm signs and last a long time.