I buy regular 8"x10" sheets of wet/dry sandpaper (the black stuff) and cut it into 4"x1" strips. I usually buy 2 sheets each of 320, 400, 600, 800 & 1200, and use about 1 strip of each on each pen (tho I often skip the 320 and go strait to 400...) I do find that I often may use 2 or 3 strips of 400 depending on the wood, so I have to replenish that a bit more frequently. I think the sheets go for 30 or 40 cents a piece at Woodcraft, which for me is cheap enough ( a couple of bucks lasts several months for me...) that I don't worry about looking for cheaper ways to buy it or feel the need to buy in bulk, and it being inexpensive, I don't have to worry about getting the most out of every piece and keeping track of half used strips...