Volatile alkali metal compounds
As Mithoron says in the comments, tert-butyllithium is one choice.  Like pure organolithium compounds generally, this is pyrophoric and reactive to many common atmospheric gases or vapors. An article by Schwindeman et al. [1] recommends using a nitrogen or argon atmosphere.

In the case of alkaline earth metals, magnesocene provides a relatively volatile magnesium compound, subliming at $\pu{100 °C}$.  This is also air- and moisture-sensitive and again, should be handled under nitrogen or argon.

References


Schwindeman, J. A.; Woltermann, C. J.; Letchford, R. J. Safe Handling of Organolithium Compounds in the Laboratory. Chemical Health and Safety 2002, 9 (3), 6–11. https://doi.org/10/ctj2sj. (PDF mirror: chemistry.ucla.edu)