I, too, shop at thrift stores for most all of my clothes. As I am a cycle-everywhere cyclist, I can find pants that fit either my waist or my thighs, but not both, so I'm accustomed to getting thigh fitters, and then taking in the waist, so they fit. The details on how to do this depend on the construction of the pants, but usually you take out the back of the waist band, then take take in the center back seam the proper amount, then put a seam in the waist band and sew it all back together. If it's a big adjustment, I might also do darts over (and into) the back pockets. I like this kind of messing around, as it takes some construction thought and not a lot of time, and the result -- voila! -- is a pair of perfectly fitted pants. I find it both intellectually satisfying and productive. Good luck on your mission!