If you have a favorite one or just can't get to the store to buy a new one, it is possible to sharpen them.
My SIL uses a bit on his Dremel machine to sharpen mine.
Also I found this at eHow.com "To sharpen your seam ripper, wrap 000 or 0000 steel wool around a crochet hook or pencil. Holding the seam ripper in your non-dominant hand (left or right) and the pencil or crochet hook in your dominant hand, move the pencil or crochet hook with steel wool wrapped around it in an upward motion. Do this to each side of the "blade" of the rippers.
000 or 0000 Emory (sand) paper will also work."