Sour oranges (aka bitter oranges, Seville oranges, marmalade oranges) are native to Southeast Asia, as are many/most citrus fruits. They're a hybrid of the mandarin orange and pumellos. Currently they're grown all over the world, at least the parts of the world that have hot enough climates. Here in the U.S. they're cultivated in Florida, and obviously the area around Seville, Spain, is another spot that produces a lot of them.
In looking this fruit up, I learned that is has many non-edible uses. Perfumes sometimes use the essential oils from sour oranges. The wood from their trees is used for carved items. Soap can be made from their fruit. Plus, sour oranges are utilized in herbal medicine, usually as an appetite suppressant or as a stimulant. (More on this later.) Not to say that they're not used in edibles, of course. One of their nicknames reveals that they're used in making marmalade. And the Belgian witbier (white beer) style sometimes uses the peel of this fruit as a flavor additive.
There are several subspecies of sour orange. The one I bought was greenish-yellow in color, and small, maybe the size of a typical tangerine, or a small orange. The pulp inside was roughly the same hue. The taste was.....absolutely wretched. Here are the notes that I wrote after eating it: "Ugh. Tastes like grapefruit--sour as hell. Awful. Why do people eat this?" I choked down two segments, then I stopped punishing myself. I squeezed some juice out of the remaining pulp, and tried that separately. This was similarly crappy. The sour orange was definitely one of the worst fruits I've tried, and probably one of the worst foods, ever. It was an even more atrocious version of grapefruit. Maybe the scent is good, or the marmalade made with it. I've liked some Belgian wit biers, so it's possible I've enjoyed sour orange in that limited context. But as a regular fruits, as a snack, or in a salad, I would only wish it upon my worst enemies, and even then I'd have to think about it. It was cheap, at least, being about $0.80 for the one I got.
As for sour oranges in herbal supplement form, there are many red flags. Evidently its negative side effects are similar to ephedra, with an increased chance of angina, ischemic colitis, and strokes. So beware of sour orange in that form, too.