It’s in loads of things. Peanuts, spinach, beans, chocolate, avocados, corn, soy… the list is endless. If someone is really sensitive, they can have problems with high oxalate foods. Some people seem to accumulate oxalates, meaning their problems compound, where others have no problem at all. Biology is weird.
Spinach is very high per serving, but all sorts of foods can have high oxalates per serving. Sweet potato, Swiss chard, almonds, beets, chocolate. You'd have to consult the content per serving for all foods you consume if you're trying to avoid them.