Thanks! to "turbo" for recommending!
To buy, search the web for "massage back roller" or for "massage ma roller", or visit your local massage / physical therapy supply shop.
CAUTION that you don't roll on something so hard that you injure yourself. The wooden roller might be a too hard for some bodies; there are alternatives. Tennis balls in a sock, for instance (old ones are softer, and preferable, imo). Playground balls, of various sizes, are inexpensive and excellent for rolling under your back. (Thanks! to Spectrum.)
ALSO CAUTION: Be extra gentle when rolling over your kidneys; the kidneys do not like aggressive massage.
"4 used tennis balls in a sock" is my favorite.
MA ROLLER INSTRUCTIONS, in Short
MA ROLLER INSTRUCTIONS, Full