The first place where you can buy retro furniture is the famous eBay. eBay is famous for hosting all the craziest items available on this planet, and it is actually said that if you have trouble finding something, you may find it on eBay pretty quick. It all works around timed options and the logged users of eBay have certain time frame to bid and win the retro furniture. eBay does gives the buyer one main advantage and that is the time limit on the auction. If you time your bid correctly and place it right in the last seconds of the auction, you can be lucky, however keep in mind that several other bidders may also be applying the same tactic. eBay also works on the silly price auction technique, meaning that if two users want the same item and they both put silly prices, the highest silly price will win.
Another place to get some retro furniture is the charity shop. Buy stuff and donate the proceeds to charity. A great and noble gesture and you can also get some great furniture while doing it. The main disadvantage here is that you are going to probably fin furniture that is from 1970, so a bit newer. However, still a good place to find some retro furniture and it is worth a shot I would say.
Yard sales are one other magical place to get your hands on beautifully crafted retro pieces of furniture. If the person having the yard sale is more than 50 or 60 years old, jump on that sale like a kid jumps on a chocolate cake. They are going to sell the furniture cheap and there is a very good chance they took good care of the furniture too.