Arm's Reach Co-Sleeper Bassinet Leg Extension Kit

To bring the co-sleeper up to the height of the bed, these leg extensions are the best bet. Yes, as someone else said, they do make it a little shaky.
So if you are not planning to attach the co-sleeper to your bed, you may want to leave the height alone. (You'll be getting up anyway, so it probably doesn't matter how tall it is.) But once attached to the bed with the anchor strap, rather than free-standing, it was very secure.