Sour oranges or bitter oranges are a type of citrus grown globally. They are often used in Hispanic and some Mediterranean cooking but in America are more commonly found in marmalade. Sour orange is the key flavor of Cuban mojo sauce– you can’t make it without it.
Because English is such an endlessly fun language to use, here’s something to keep in mind: while sour oranges and bitter oranges are the same, orange bitters (like those you would find bottled in a liquor store) are not. Don’t try to use these cocktail bitters in a recipe that calls for sour orange / bitter orange. It’s not going to work.
If you live in a place that produces a lot of citrus fruits you may be able to find these in the grocery store. I have had exactly zero luck finding fresh ones in Colorado. However, you can often find bottles of sour orange marinade in the Hispanic section of your grocery store or in almost any Spanish market. Check the ingredients- they can vary widely but your best bet will be any one whose first ingredient is actually sour orange juice. The best one I can find around here (although I admit to not being totally married to it) actually has grapefruit as its first ingredient. This is the Iberia brand shown below. Of the brands that I am able to find locally, this is my favorite; but I would LOVE to hear any suggestions for a brand that other people prefer.
I strongly recommend treating yourself to the real deal if you can. If you absolutely can’t find any sour oranges or sour orange marinade, you can make a reasonable substitute out of equal parts orange juice and lime juice. Simply combine and let sit for a minute before use!