I will only buy new pants in the snuggliest size that I can button and zip (or almost button and zip).
Otherwise, I buy at our local thrift store. I buy mostly 14s now, though I still wear a 16 or two depending on cut and fit. They let you clothes that still have tags attached (if I shrink too fast), and they buy back clothes you have worn and shrunk out of.
I can't tell you how many times I buy a couple pairs of pants and return half of them within a month or two.
Plus, everything is WAY cheaper than buying new. I save my 'new' money for something that really fits well - like my brand new pair of size 14 button fly booty-pants