MANY out there on the internet-you can even stand and learn how to contract all muscles,google away!
Try sitting still and just tighten and release stomach muscles like you are sucking it in. You can do this throughout the day while you work or watch TV what ever.
Hula hooping! Also yoga can be great for tightening all areas of the body
http://www.livestrong.com/article/523676-good-stomach-exercise-if-you-have-a-bad-back/
Have you tried doing the plank?