I shop in thrift stores, but am a little bit of a fabric snob. I don't like to buy from the thrift store as I don't know how the fabric was washed, or even if they take the time to wash the fabric before I buy it.
I do have that problem at garage/ estate sales. If I see fabric at one of those I have a very hard time NOT buying anything. I feel more comfotable buying there as usually the previous owner is there and can answer questions about what I'm buying.