Vilene Standard Firm Iron on White interfacing.
 
This is what I use to stiffen up any fabrics/felts before I stitch, especially used in free motion embroidery and dressmaking to stiffen up parts and make the fabrics easier to manipulate.
 
Sold by the metre. 90cm wide. 
 
Shinier side is the side which you gently iron onto the reverse side of your fabric.