The U.S. Department of Defense’s ASSIST (Acquisition Streamlining and Standardization Information System) website is the official repository. A direct search for "MIL-STD-167-2A" will yield a downloadable PDF free of charge, but note that access may require a CAC card or registration for contractors.
: Cover internal and external vibrations for rotating, non-reciprocating machinery. mil-std-167-2a pdf