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.
It turns out that some foods counteract the formation of kidney stones. If you drink milk when you eat oxalates, the oxalates will not fuse into stones and will be excreted normally in your urine.
Oxalates are present in tea too, though not coffee, IIRC.
Spinach. Many nuts, e.g. almonds. Many beans (including cocoa but excluding coffee). Well, potatoes!
Spinach. Maybe other leafy vegetables.
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.