The authoritative sources for physical constants are the 2006 CODATA
recommended values, published in the article below. Further
information on the values of physical constants is also available from
the NIST website.
P.J. Mohr, B.N. Taylor, D.B. Newell, “CODATA Recommended
Values of the Fundamental Physical Constants: 2006”, Reviews of
Modern Physics, 80(2), pp. 633–730 (2008).