I recently got some wet/dry sandpaper in 220 and 600 grit. Pretty darn cheap. I lapped a 12K Naniwa and it worked quite well. First did 220 then did again at 600. I found sandpaper easier to use because of its huge surface area compared to any lapping plate.
seconded,
I used 3M wet/dry sandpaper 220 as well if I recall correctly, worked wonderfully