I've searched for "plant" & had success finding stuff, usually garage sale people that only have a few but sometimes I can find a whole load. There's usually a lot of fake plants to weed through though.

I've seen plants listed in the farm and garden section, as well s the free section on Craigslist. You have to be quick to get the free stuff though. Try Freecycle too.

you wont find too many plants in the winter. in the spring and fall when people are dividing/planning/cleaning theres tons of giveaways tho.

I scored 58 (!) free boxwoods on Craigslist, although I will admit that's unusual. The lady listed them in the "Materials" section and I just stumbled across them. Dorisl is right, you'll do better to keep your eyes open starting in the spring. (I had to dig out the boxwoods in July, which wasn't ideal.) First thing every morning, when I check my email, I do a quick check of the "Farm & Garden", "Free", "Materials", and "General" sections. I've often seen offers of shrubs people will give away if you come dig them out.

Good luck!