Where is the best place to buy a Cockatiel?

Look for breeders that enter competitions. You can always buy a bird that is not up to competition standard, but would otherwise make a good pet. They will be happy to sell such birds, knowing that they will go to good homes. We did this for our dogs, and we were not disappointed. However, we did have to agree to two of the dogs being neutered, so that we could not use them for breeding purposes (their sire was a champion). I'm not sure if that's possible for birds.

As an alternative to Competition-breeders, check out rescues if any are in your area. Often, competition-breeders, hobby-breeders and rescues are networked with each other anyway, so finding one in your area should open up a whole lot of contacts!
Cockatiels are a lot more specific than, for example, rabbits, guinea-pigs and the like, but I suspect the networking between interested people is the same.

There are several options. (1) Check local shelters for rescued birds. (Most rescues got out of their homes and, because they had no idea what the outside of their houses looked like, couldn't find their way back home.) (2) Check for breeders in your area. A web search for cockatiel breeders or pet classifieds-birds may turn some up. (3) If you live in a metropolitan area, see if there are is a bird mart in your area. (Again, do a web search for bird marts and your city name.) Breeders gather at bird marts.

I always prefer adopting birds or getting them from breeders. In the former case, you'll be doing something very good. In the latter, you can meet several birds at once and learn about their histories. We don't understand what makes birds choose their people. At a breeder or a bird mart, though, there will often be a bird who can't take his or her eyes off of you and who keeps moving toward you. That bird has chosen you and is likely to be a great companion.

I have two cockatiels on my shoulders as I write this. One is a rescue bird and the other was from a breeder and chose me. Best of luck, and enjoy the cuddles once you get your bird.