Fabric medium is a liquid that is applied to cloth to enhance its ability of accepting paint. See link. http://www.ctpubblog.com/2009/11/06/how-to-improve-your-fabric-painting-with-fabric-medium-and-gel-medium/
One would assume any good quality craft store would have this type of product.
You can get medium or if you already have paint you can get additive. It is sold by most places that carry decorative paint ie: Plaid, Ceramacoat. (Hobby Lobby, JoAnn's, Walmart) Look with the colors of paint -usually above or beside along with extenders and glazes.