Nick, here are the Usual Hiding Places for standard reduction potentials:
Handbooks. The CRC Handbook of Chemistry and Physics  is the first place to look (the zinc reaction is there). If you don't find what you need there, other general chemical handbooks have more extensive tables. For example,
check Gmelin (Gmelin Handbuch der Anorganischen Chemie, 8th ed., New York, Springer Verlag GMBH). Gmelin is a compilation of almost two centuries of accumulated data about inorganic compounds in a 500 volume set. It isn't available online; you'll have to make a trip to a major research university's library to search it. Gmelin contains most of the standard reduction potentials determined.
Textbooks. Any upper-level undergraduate inorganic chemistry text has a table of standard reduction potentials (e. g. the manganese dioxide reaction is in Advanced Inorganic Chemistry: A Comprehensive Text, Interscience, New York (1966), p. 835).
Reference books. You can find reference works devoted to standard electrode potentials at your nearest university library. For example, look for Dekker's Standard Potentials in Aqueous Solution (1985).
The Web. I don't usually bother looking on the Web for data like this, but I did find standard reduction potentials for both of your reactions on Mark Winter's comprehensive WebElements site. Dr. Plambeck at the University of Alberta maintains a small table of E° values, which includes the temperature dependence.
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Author: Fred Senese email@example.com