I was fortunate and recently picked up enough yarn at a thrift store to make a sweater or a small blanket, or whatever. I always look in the thrift store's craft section when I go, and once in awhile I get lucky.
As to the stash, I'm almost ashamed to say, mine keeps growing!! I started seriously knitting a little over a year ago and while most of what I have is bought with a particular project in mind, it will be years til I finish them all. My husband is already complaining it will be 10 years before he sees the sweater I've promised him!
I'm with the other gals as to not taking a project with you to someone's home that you are not really good friends with. There are a couple of gals at the hospital where I work that often bring knitting with them to a meeting. I think that's pretty rude from the perspective of those who are running the meeting.