This maybe a common sense but...
1) try to get the healthy fish(no dead body in the tank, not too skinny, no fish in the tank have icky white spot, etc.)
2) try to get from the reputable store with knowledgeable stuff( sometime big chain fish store may not be good)
3) ask questions before you get(TIME like how long they are in store so you know when they were in the stressful shipping, and probably better at least week after that. so they had some time to recover.) (the LOCATION of origin. I don't hear much good thing about Asian origin, now florida is getting iffy. So I prefer to buy Neons which were bred and raised by other hobbiest if you can find. If you are living in big city, check the local aquarium society.) (parameter of WATER they were kept, temperature, ph, gh etc,. Try to much as much as possible or acclimate slowly but no sudden change please) (GURANTEE some store offer the gurantee for the fish purchase so ask how long and what)
4) try to get the younger fish so you have more control of their health. You can grow them healthy stronger fish.
I think all the things I mentioned above is good to remember when you are getting new fish not only the Neon tetra but all the fish.
And I didn't have any problem with my Neons. I bought from small local store. They were about 1/2". Not long after they colored neon. And I grew them. I believe they were from Oregon which maybe originally from Florida. Anyway, after few months they are now adult size. I'm about to set the breeding tank. And when I get the extra frys, I will offer to my local aquarium society. Of course, I will keep as many as I can keep since they are my favorite fish.
And younger fish(small) are usually cheaper also. And you have longer time to enjoy also.
Anyway, I hope you al enjoy the Neon that they are pretty little peaceful fish which is hardy once they settle down. And I think also that they prefer heavyly planted tank with some open swimming tank. Place the opening space in front so you can see them often.