One of my favorite fabric stores is the local Goodwill Store. Someone donates her lovely silk clothing there every now and then, so I like to go every few days to grab up these gems. I've found blouses and skirts, even a wedding gown, and this past Wednesday I found a nice set of silk chiffon pajamas.
I "de-seam" them and use the pieces for purse and pocket linings; but mostly I give the pieces to my class of beginner sewists' to practice working with delicate fabrics. I especially look for stained and torn clothing that would go unsold. Often they will give you a further discount if you ask (because the money goes to a good cause, I usually don't ask with small items, just on more expensive items like formal wear). This keeps costs low for everyone and reduces the landfills' loads a little more.