I use Jim Gremel as my source as he is local, but also goes to many shows and conventions.
I follow a different rule of thumb than mentioned above. Copper wire is used for evergreen, and aluminum is used for deciduous. The reasons being that copper is tougher (but still easy to work with), holds better, and is more expensive than aluminum, and will stay on a slow growing evergreen tree much longer. With deciduous trees you will often be wiring more than once per year, removing the old wire as the tree grows, and repeating this process.