Google a company called "The Andersons" look around a bit they make the media,several different screenings. I use the finest one they make as it does not stick in flash holes. 2 Dealers in the GTA. About $34 fo a 40lb bag. I add 3/4 of a cup of mineral spirits and a good squirt of Turtle wax(large Dillon Tumbler 40 cup capacity), run the tumbler with just the mix in it for about 1/2 an hour. Works better than anything comercial and is way cheaper.
Andy