Leopard Gecko help?

Hey, so I use to have two leopard geckos, they both unfortunately died I got them both at Petsmart which I now realize was probably a mistake. They each died of the same thing. The first one, I was a little late in getting him to the vet. And the second one, after several trips to the vet, still died. They both died with very thin tails and they were very lethargic. I had the temperatures in the hot side around 90 degrees and on the cool side around 70. I had a shedbox that had fake moss in it and I misted it everyday. I had a water dish in there and two hides, one on the hot side and one on the cool side. I also eventually put a small dish of calcium dust in the tank for my second leopard gecko because I was getting worried because his tail was getting thin. And I did dust the mealworms with calcium dust. And I switched from crickets to mealworms because my second leopard gecko wasn't fast enough to catch the crickets and they would end up just hoping around the tank or bugging him. I fed both of them everyday, the first one was just on a diet of crickets and he ate about 4 or 5, my second leopard gecko ate about 5 to 6 crickets and when I switched him, he ate around 3 mealworms. And both leopard geckos were on reptile carpet. Oh and another weird thing, my first leopard gecko, about a week before he died would just sit in his water dish. I just want to make sure that it was just bad luck and not something I did..So now to my main question, would it be better to buy a leopard gecko at a reptile expo? There is one coming up in about a month. I would like to get another one but I am really scared that it was something I did that killed my past two leopard geckos

It sounds like your older geckos could have had crypto.
I would highly recommend getting a gecko from an expo if you really want another one. You can directly ask the breeder how they take care of the geckos and get tips and advice. I would also recommend buying a gecko that is a healthy adult.
I dont know if you were planing on using the old supplies for the new gecko, but I would get rid of everything that came in contact with the old geckos and just buy an entirely new setup for the new gecko. You can find great deals on setups at the expos.
It does not sound like the geckos deaths were your fault, it sounds like they had some sort of internal parasite that they either got from the petstore or more likely a bad supplier.

I agree with the thought of Crpytosporidium.

Please do not use anything AT ALL that you used for your previous geckos. Crytpo spores can last for YEARS unless cleaned with a 5% or higher ammonia concentration or a sterilizing UV light. There is rumor that hot moist heat (say, putting it through the dishwasher with heated start and dry) would do the trick, but I like the idea of UV or being doused in a high concentrated ammonia better.

Get all new supplies, feeders, tools, everything. You do want any risk of the new geckos who you purchase healthy to die because you reused anything the crypto infected geckos were around.