I know there's a lot of hesitation when it comes to Craigslist but for yarn buying it can really be your friend. It's very hit or miss but when it's a hit you'll be glad you made the effort.
This morning I got a kitchen sized bag full of yarn for $29. Maybe a shopping bags worth as well because I already shoved the crochet thread in it's tote. The kitchen bag is everything else I got. I think about half of it is pure cotton skeins. The rest is novelty yarns, wool blends, a couple pure wool and some cotton blends. I got only one pure acrylic and that's because it's the mohair look by Jiffy and I've been wanting to try that. Not that I'm prejudice against acrylic, but it seems my customers are.
Previously on CL I've gotten yarn for 25 cents per skein and once I got a kitchen bag full for free. Admittedly things don't always work out so great. I've paid $2 for a couple acrylic skeins because I felt awkward driving all the way out there and not buying anything. Other times the person said they had loads of yarn at their garage sale only to have it be a shopping bag sized pile of half used acrylic skeins.
I check CL several times a week using the search keyword 'yarn'. The search will return all sorts of things like carpet and marimba mallets. There'll also be tons of hats, scarves, etc people have made and are trying to sell. I'm pretty sure you're not supposed to use CL for this sort of thing so I ignore them all. If you want homemade goods, shop Etsy.
After sorting out these you'll come across some for people destashing or cleaning out their storage unit or garage sales. I only bother looking at ones that have a picture. Then you can decide from there what's worth driving to look at. Sometimes you get people that are dreaming of getting their money back. I really don't bother with anything more than $1 for a new skein. I know there are deals that are better than retail but that's not my goal. With patience you come across the fire sale type deals.
Now if you live out in the middle of nowhere this probably won't work for you. But I imagine those folks aren't reading anymore. If you're near any sort of decent sized city, just be patient and keep checking. Eventually someone will move and not want to take all their yarn. Or a relative will inherit a pile of yarn they have no idea what to do with. If you check regularly it won't take as long because you'll recognize all the ads you've already rejected by title.
So now I've got tons of new project ideas forming and dozens of skeins to ball so I'm off.
Good luck and happy hunting!