i just bought a fine one today at home depot...
thing is, the one i use (6") seemed OKAY. but tore it up a little easy.. i thought it was a coarse one..
so i bought a fine one.. took it out of the package.. and feels the same.. so i wanted even finer..
i found this, and ordered one today.. should have it next week or so..
http://www.leevalley.com/us/wood/page.aspx?c=&p=65204&cat=51&ap=1