I know that spectroscopy can be used to analyze the light of stars to determine the elements causing their emissions in the visible spectrum, but this requires knowing the emission spectra of the elements themselves.
Is there a standard source for this data?
The chemical composition of stellar atmospheres is usually obtained from absorption lines.
The analysis ingredients are a good spectrum, a model of the temperature and density structure of the stellar atmosphere and a catalogue of atomic and molecular transitions. It is these latter things that I think you seek.
Such "linelists" are found in many places; consisting of line wavelengths, excitation potentials and oscillator strengths.
One very useful compilation/database is the VALD atomic line database. Other catalogues exist for molecular transitions -e.g. Tennyson & Yurchenko 2012. However, these lists are usually just the starting point. Most spectroscopists tune their lists by a careful comparison with the spectrum of the Sun, or other standard stars.
