My local Ace Hardware has some nice spacers too, out of various materials, including steel and aluminum, in various DOM.
As for where to source the alternator, you'll want to do some research before purchasing one as there are many different housings and mounting points available for the 3G alternator. Stock 5.0 aluminum bracketry can be found inexpensively in junkyards and can be made to look like new with a little tumble in a deburring machine. The serpentine upgrade with automatic tensioning is really worthwhile.