There are so many bed skirts available in thrift stores for all bed sizes, I buy them all the time. The prices are usually under $5 even for king size and since a bedskirt gets very little wear and washing, they are usually fairly like new. So you can change them out on your bed often. Also, bedskirts work great as fabric to use in borders, kaleidascopes and other applications where you need long areas of yardage you don't want to piece.
That way, your quilts can just cover the mattress and you can always have a nice cover for the springs.