If price isn't a major concern (i.e. you're willing to pay prices only slightly lower than new furniture prices for antique and older furniture from the era when it was all made of real hard woods), and want to just buy something good quickly, antique malls are a great choice. This will be retail prices, which is at the high end of the old furniture pricing scale, but you can walk around and see dozens or hundreds of pieces in an afternoon of shopping. Also, most of the time, the items will be in good usable condition right off the floor; my mom, when she dealt mostly with furniture, would restore/refinish/re-upholster anything that needed attention before she put it in her booth (as long as that attention wouldn't hurt the value, by stripping away something intrinsically interesting about the piece). You can also often get a small (10-15%) discount just by asking. You can make offers, but they'll have to call the owner of the item to make any sale that isn't pre-arranged (my mom, I think, has a standing instruction with the folks who run the antique malls where she sells to allow a 10% discount if the person is buying with cash right then, repeat customers may have even better deals available to them).

If you want to get a bargain and can invest time, my mom buys at garage sales, flea markets, thrift stores, and auctions, in that order (with garage sales being, by far, the best place to get good prices, but also the most time-intensive). Estate sales (a sale after someone has died and their entire house is being emptied out for sale) in old neighborhoods are the best option for finding antiques and old furniture. But, these days, even estate sales may have no good furniture. The first generation to start buying mass-produced trash furniture is now beginning to be among the folks dying and leaving an estate to be dealt with by their family.

There are also often antique expos in most major cities, where lots of dealers come from surrounding cities to sell; happens a couple times a year, generally. Again, this will be retail prices and maybe even higher, but you have a lot to choose from. If you're looking for stuff that doesn't sell well, you can drop in at the end of the last day and make offers on stuff you like...they don't want to load it up and carry it back home, so they'll let it go for a song. As recently as a decade ago, I was able to buy some really nice Mid-Century Modern stuff at a good price this way (good enough that I made a modest profit when I moved a couple years later and sold those pieces on craisglist).

But, I usually buy on Craigslist. It allows me to setup searches for specific items I want, and I can make offers when things come up that I like, even if the price isn't quite right. People on craisglist may or may not know the right price for something, and often just want it gone. For mass-produced stuff, you can often figure out what the right price is by searching on eBay for recent "Sold" listings. This is not as cheap as garage sale prices can be, and if there is a bargain you have to jump on it immediately, but you don't have to invest a lot of time or wake up at the butt crack of dawn to find good stuff at fair prices.

Unfortunately, craigslist is in decline, so you also may want to check Facebook marketplace (which is a usability trash fire, and I hate it, but lots of people use it). It's nowhere near as usable, in terms of saving searches and getting notifications about stuff you might want, and it doesn't really have good filters like craigslist.

Speaking of getting deals, when you're in the market for stuff like this, carry a nice chunk of cash with you everywhere. Paying cash can get you discounts with a lot of the folks who sell antiques. They aren't often technically savvy, and their credit card fees are often very high (like 3-5%, and the venue they sell in may even add some extras on top for card transactions). And, while most professionals in the business are keeping good records and paying taxes on everything (and malls definitely are), a lot of people for whom antiques and collectibles is a side hustle, as you find at flea markets and expo events, will be happy to have some money that's not on the books.