These values (will) have been aggregated from a variety of sources (once we add more).
Vacuum, helium, neon, hydrogen (H2), argon, air (dry), nitrogen (N2), water vapor (100 C), benzene, diamond, salt, silicon, methanol, water, and ice (-30 C) all come from table 4.2 on page 187 of Griffith's "Introduction to Electrodynamics" 4th ed. All values are for 1 atm and 20 C unless otherwise specified.