Do Seville oranges have an American equivalent?
Not that I’ve ever heard of or seen. Every once in a while I see imported Sevilles, and you can get “generic” sour oranges here aka “naranjas agrias” (in some places, in season), but they rarely have the thick rind that’s key to making good marmalade.
I dare say sour oranges (if not necessarily the same varieties sold as “Sevilles”) would grow in the limited areas of the US where oranges can be grown at all (and to generalize, or probably overgeneralize, the sour varieties tend to be cold-hardier than sweet oranges), but I assume they’ve never been as profitable a crop as sweet oranges, so aren’t in fact grown here in commercial quantities. I don’t know where the ones sold elsewhere in the US come from, but the ones we get on the East Coast are almost always from the Caribbean (mostly the Dominican Republic in recent years), where they’re used more for their juice than the rind, which is no thicker than the rind of typical “juice oranges”. Fwiw, it does have a distinctive aroma that I like better than regular orange rind - it’s a little more complex and less “monolithic” orange…
ETA: Having looked through the Chowhound thread, apparently Seville types are available out West, or seemingly so, but it doesn’t look like they’re grown on a large scale, nor distributed very widely… and never having seen them myself, I’m not in a position to say whether they have the typical thick rind of the “original” Sevilles (similar to navel oranges, as someone else in that thread confused them with), or the thinner rind of juice oranges.