[THX_138]

You may need to do some paint stripping or creative cuts, but it is amazing what is right under our own nose sometimes.
Sliding glass door chassis/frame holds an abundance of aluminum.
Channels from double pain windows.
Top work surface from old broken table saw.
Many types of old office furniture.
Unwanted or broken aluminum ladders is a great source.
Some lawn furniture.
Even some BBQ grills have aluminum frames.
A simple square tube sliced down the center can end with some sweet frame rails.

I do buy at the store when I can, but the things that have reprocessed materials in them give me the greatest satisfaction.