Nothing beats free: With that in mind here are the five most relevant places to get free stuff.

Hopefully theres a few places you may not of thought of.

1. Facebook Marketplace: Facebook Marketplace has a free section.

2. Craigslist:
Craigslist has a free section.

3. Just being social:
Chances are your already spending at least some amount of time on social media. Pay attention to what people are up to.
If they're starting a project be aware of what might be becoming available.
If there taking down a shed offer to help in exchange for some free used lumber you could put to good use.
These sorts of things are always happening so stay awake.

4. Put the word out to the right people:
If there's something specific you know your going to be in need of let those most in the know.
By sheer chance my neighbor makes part of his living taking down trees.
When we first moved in he and I hit it off immediately.
 He's a super good guy we immediately knew we could trust.
With two fireplaces to feed, he's already given me a couple of free loads of firewood.
 So it goes without saying it always a good idea to let people in doing certain work about certain items they may likely to come into contact with.
Tree trimmers can clue you in to some free upcoming firewood. People at garden centers know when those five-gallon plastic containers are to be thrown away.
 That construction worker you know can tell when some free fill dirt becomes available saving you hundreds of dollars.
 Or maybe some discarded cinder blocks.
It's a GREAT IDEA to have your contact info to be able to quickly give someone, i.e. business cards or pre-written down cell phone etc.
What were really talking about here is networking.
Some people have made fortunes networking.

5. Flea market trades:
By sheer chance, we live about four miles from whats probably the biggest flea market in the county we live in. It's an all summer event but only on Sundays.
 Everytime I've been there theres literally dozens of vendors you know you could trade stuff around the house for some specific item you might be in need of. 
This is especially true when it comes to things like tools, books, and knick-knacks, but not as true for other things like food or some collectables.
 But sometimes it does pay to talk to people.
It never hurts to talk.