I've got four granite surface plates, and they are all utilized in different parts of the shop. For this type of work a cut off from a granite counter or a granite floor tile is plenty flat enough and cheap. I have one granite that I use for this type of lapping and depending what is on hand I'll either slop some oil between the paper and stone to hold it in place, or use a light coat of spray adhesive to hold it on. The paper moving around is a PITA. For steel I like a light 10W oil and for aluminum I typically use WD40 in a squirt bottle. Random pattern or figure eight is important.
