Aspartic acid is a non-essential amino acid. In plants and microorganisms, aspartic acid is the precursor to several amino acids, including four that are essential: methionine, threonine, isoleucine, and lysine.

Aspartic acid is found in both animal sources (luncheon meats, sausage meat, wild game) and  
vegetable sources (sprouting seeds, oat flakes, avocado, asparagus).

References

IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. Nomenclature and Symbolism for Amino Acids and Peptides. Recommendations on Organic & Biochemical Nomenclature, Symbols & Terminology etc. Retrieved on 2007-05-17.