How to make/preserve popcorn so that its taste and texture doesn’t deteriorate within a day or two

I think this is primarily going to come down to the method of cooking and the flavorings. I think it's unlikely that you can buy a microwave popcorn that will taste like the movie theatre.

You'll need a specific popcorn pan so that you can stir the popcorn continuously and that will also vent some of the steam from cooking (which a normal pot won't do. Whirley Pop seems to be a common solution here.

Next you'll need to find the right type of oil. Experiment with a couple of things to see what you like. Coconut oil seems to be commonly recommended (and not easy to find).

Finally you'll need flavoring. Try Flavacol. My work has popcorn on thursdays, and they use this flavoring. It's pretty dead on.

I don't think the actual pop corn kernels will make a ton of difference. It's more the cooking technique and other ingredients.

Here's my primary source, but further research and experience backs that up as pretty dead on.

Round popcorn is just made from a different variety of popcorn, called mushroom popcorn. The "normal" kind is called butterfly popcorn, or sometimes snowflake popcorn.

The two are mentioned in the wikipedia article (last paragraph of the linked section), along with a photo. It looks like it's pretty easy to buy online, if you prefer it.

Mushroom popcorn kernels are a little sturdier and easier to coat, so it's well-suited for prepackaged popcorn and especially things like caramel corn, but at least to me that sturdiness is also a slight toughness, so I prefer the butterfly-style popcorn most of us are used to.