I've gotten yarn from
freecycle.org - I posted that I needed yarn to make charity items and I got a very large bag of misc yarn (it smelled of cat urine, so I machine washed the yarn in a vinegar solution, and then it smelled wonderful) (free isn't always perfect)
and
craigslist.org - I search for "yarn" and I sometimes find some good deals after haggling (I once got 7 skeins of pound of love for $30)
and
yardsales.net - I search for "yarn" and once in awhile I'll find something (last week I got a large tub of yarn for $25, it included 2 pounders, 6 variegated, 8 homespun, 10 cotton and some misc)(that was my jackpot!!)
It takes awhile searching online. I think I get good deals about 4 times a year. I enjoy traveling and going to yard sales, it's my hobby too.