I use these Neiko flap disks from Amazon, 20 for $25
http://www.amazon.com/gp/product/B008DJGWPC?keywords=60%20grit%20flap%20disc&qid=1457480552&ref_=sr_1_2&refinements=p_72%3A1248921011&s=industrial&sr=1-2

They are pretty good for the low price.