Corona is one of the classic Battle Armor designs, it's widespread among all Clans and is a staple product in Sea Fox (and others') catalogue
Literally none of this is accurate. It's not a "classic" design (it only hit production in 3060 and was only used by a few Clans at that time) and was never "widespread among all Clans", it doesn't appear on a single Clan's RATs in the Dark Age/ilClan era with the sole exception of the Scorpions (the RATs aren't the end-all-be-all, yes, but between them and the production facility listings, they're an excellent starting point), and it certainly isn't a "staple product" of the Foxes because they don't have the ability to produce them. No one does beyond the Star Adders and
maybe the Cobras.
Do they exist in some form in the Inner Sphere? Of course they do. Do they exist in enough numbers that they're being sold by their owners to interested parties? Not very likely, given their rarity. Are they being produced by anyone outside of the Adders? Not that we've seen thus far. But like anything else, they can be salvaged or taken in a ToP, and there's room for a one-off sale here or there.