Estate sales
FaceBook Marketplace
Thrift stores
FaceBook Buy Nothing Groups
Freecycle
Craigslist
Salvage stores
Flea market
ebay
Habitat ReStore--great for wood, pipes, and furniture
TJ Maxx/Ross/Tuesday Morning, if you're in the US
Curbs on big trash day (look at your city's website to find out when, and I'd focus on the ritzy neighborhoods and college areas near the end of term)
Gift wish list--if your birthday/major religious holiday/milestone event is coming up, register on Amazon or Anthropologie or wherever you find the perfect item, and let everyone know that you have your heart set on item XX and you're saving up for it. Some of my friends eventually got their coveted Pottery Barn couch by telling everyone that they were saving up for one. All those $20 & $50 gift cards can add up.
Gift card arbitration--find the perfect item at particular store, then purchase discounted gift cards from Ebay, Gift Card Granny, or other aggregator sites.
Figure out how you want your space to look, then visit some of these options to see what's available. Pinterest is great for that.
Word of mouth. Mention to friends, coworkers, acquaintances, anyone, that you're looking for an 8x10 rug with some black and white. You might be surprised and be offered several. You never know whose MIL gifted them a rug they hate, but it's great for your space.
In my case:
My solid oak antique Victorian-style dining room table was free from a friend who was moving and didn't want to transport it.
The chairs are antiques from a flea market, $29/4.
My two matching West Elm loveseats, solid wood coffee table, and matching hutch were $600 on Craigslist because the seller treats himself to new furniture every two years and just wants it gone. He even threw in a framed movie poster that I later sold for $30.
The 8x10 wool living room rug was $40 on Craigslist.
My side chairs were off Ebay for 2/$99, including shipping.
The two original framed paintings were 1) free, for helping a local artist with her show, and 2) gifted to me from a friend who got me another of her works after I raved about the first.
Hope this sparks an idea!