Linen-N-Things may have some, they have an online store too. Also catalogs such as Lillian Vernon and Miles Kimball sometimes have a set of elastic to keep fitted sheets snug. They are a few inches long and each end has a clip-one clip on one side of the corner and then diagonally across the mattress corner to the other side of the same corner
http://www.mileskimball.com/mileskim...78&icmp=search
oops just saw trish b's comment