Healthy/egronomic backpack needed?

Can anyone tell me where I can buy an inexpensive backpack that will be "healthy" for an aging back? Also, please suggest one that maybe you have used. I suspect that my lower back pain is due to the fact that I still use a backpack (at age 34 - and, no, I won't use a briefcase). I'm also going to go see a doctor, but I figured that it is also good to get a backpack that is good for the back, anyways. Thanks!