The Yarde Metals Drop Zone is currently listing several sizes of 0.125" 5052 aluminum sheet available:
http://yarde.com/cgi-bin/dropshop.pl...size1=++0.1250
Logan Steel in Meriden/Wallingford also has a surplus/scrap area which often has a lot of aluminum stock.
McMaster also has a wide variety of raw materials, but their prices on aluminum stock can be a bit steep compared to other options.