You can go to most arts and crafts supply stores and pick up some silk screen material. It's no longer made from silk, but a more durable nylon mesh. You can get it pretty much any size you want, but, like most fabric, it's sold by the linear or square yard.
I'm a former screen printer and printing is a mass production process. It takes several hours to set the job up. All of that work isn't worth 1 t-shirt, especially if it is multiple colors. The price goes down per unit the more you buy. $15 is a steal if this is a one of a kind shirt you are asking them to print. I wouldn't touch a one off job for that.