Yes, I know Neon got some bad reputation. I blame some of it to the South Asian mass produce operation. You know they sell those Neon for several pennies each if you buy like a thousand of them.
Anyway, if you are looking for high quality Neon, you should avoid one of those big chain pet store. Odds are, they are getting Neon from S Asia. Or sometime from Florida. Either way, they can't be strong. It's a simple economic that fish value is not high, they can't and won't spend money to raise them. It is a sad situation but that is the way of Neon.
I know Neon can be hardy if you buy from the reliable souce. Maybe you can find some from aquarium society in your city. They usually have meeting and auction sometime. Or locate the local breeder near you. Although this is really unlikely since Neon's price is so cheap that it just not make sense to breed for profit especially considering all the trouble raising tiny tetra frys. But some aqualist do for the sense of accomplishment. Also if you are lucky, your neighbor small local pet store got them from some hobbyist who bred them.
And try to get the young small one before they grow up in the mass chemical dosed environment. It is better for you to grow them to adult. It won't take long anyhow.
And if you are able breeder, you can get the 2nd generation. And they would become much hardier since they would be under your attentive care. As long as they do not have some genetic weakness.
I have my 2nd generation in my tank and they are doing fine. I should make 3rd generation but don't have time.
So get the high quality Neon and they would be much hardier than the one you find at the giant pet store chain. Also ----- tstenback ----- 's suggestion would help. You can always ask the cleck of the store where they are getting the fish. Although not always you get the answer, some simply do not know.