Thrift stores, basically. I worked in various thrift stores for 5 years and mostly furnished my home that way. Just be thorough when you're inspecting it before you buy it! Check for bugs, broken/missing pieces, wiggly legs, cosmetic damage. We've gotten some really cool stuff in great condition for actually reasonable prices. My favorite is an ancient La-Z-Boy for $12, in decent shape.

If there's a Re-Store near you, they're great too! We got an adorable coffee hutch for $65, a decent couch for $80, a couple nightstands for $10-20 each, a solid, heavy duty coffee table for $30. It requires patience to find what you might be looking for, and it helps if you already know which thrift stores tend to get better quality items. Antique stores are worth a shot too, if you're ok with spending on the upper end of cheap? You can get some really well built, old, beautiful furniture. Or see if there's a furniture consignment store around you!

I think things like IKEA or Target are fine too, as long as you take care of your furniture. I've never had any problems with anything from IKEA, even when I buy used, but it probably helps I don't have kids, lol.