I don't accumulate many plastic bags myself, as I generally shop with reusable bags (they're so much nicer, anyway). So in order to make my plastic bag crochet projects, I gather used plastic bags from family, friends, and strangers (via Freecycle).
I'm given a lot of newspaper bags. I believe they're from the Houston Chronicle. Apparently, the Houston Chronicle is delivered in a different color bag every day--many soft shades of gray, green, blue, mauve, and more. I've accumulated quite a stash of these bags. Check it out.
The photos don't really do the colors justice. I think it's my cheapo camera. Still, aren't these colors great?
My favorites are the greys. I love grey. I've been a big fan of grey for the last fifteen years. (And yes, I spell grey with an "e" even when my spell-checker argues with me.)
I spend way too much time looking through these bags and coming up with the best color combinations for my next projects (and not enough time actually getting to work).