Are there any VHDL source code libraries available to save me having to re-invent common code fragments and functions?

There are a few libraries available for most levels of VHDL design. The IEEE library contains very low-level type-and-function packages. The std_logic_1164 package is an industry standard, and practically every piece of VHDL you ever write will use this package; the types std_logic and std_logic_vector are the overwhelmingly dominant types for anything related to digital design. For arithmetic, numeric_std (from the same IEEE library) is a collection of functions that work on std_logic and its derivatives. For other libraries of components, have a look in the comp.lang.vhdl FAQ.