There are several versions out there. I have no firsthand experience with any of them.
http://www.the-backup.com/
http://mattressholster.com/djstudios..._holster_3.htm
http://thebedsideholster.tripod.com/
http://www.pillowholster.com/design.php (Interesting. Never saw that one before!)
There's another one that I can't seem to locate, but it consists of a long, flat "board" that slips between the mattress and box springs and the holster is attached to the part that hangs down on the side of the box springs.