As a general rule, this information can be obtained from either searching for the aircraft involved (it's amazing what you can find), but the more hard core flight dynamicists will invest in model-specific books and manuals.
A good place to start is usually the FAA type certificate site (when moving to this new computer, I lost the URL, but generally start at
http://www.faa.gov), that provides the basic performance characteristics.
Many times, by inputting the "basic" numbers into the air files, you get a reasonable approximation of the handling, but reading descriptions, manuals, etc, help out.
Of course, in the case of obscure aircraft with little available documentation, you have to rely on "Scientific Wild A$$ Guesses" - calculated estimates based on good faith and "what feels right".
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