(sarcastic answer removed)
A quark is a subatomic particle. They are found in protons and neutrons, for instance, which together make up the nucleus of atoms.
The word also appears in Finnegan's Wake, in the line "Three quarks for Muster Mark!"
The answer is twofold: everywhere, and nowhere.
A quark is considered the most elemental of all subatomic particles. Quarks make up other subatomic particles, such as protons and neutrons. Because quarks make up other subatomic particles, and because all matter is made up from subatomic particles, you will "find" quarks in everything you see, touch, smell, and taste.
First, quarks are too small to see or measure. Therefore, you could never actually "find" a quark. Second, quarks always - but always - exist in combinations with other quarks to make up other particles. Quarks do not exist alone as free quarks. Therefore, even if you could see them, you could never find just a quark.