Out here I would buy it at a place called "Blue Collar Supply" where they sell all kinds of Aluminum by the pound.
Every large city will have places to buy Aluminum. Lowes out here has a small selection of Aluminum and you may find what you need. The problem with them is they think it made of Gold and may not have channel or Angle that are heavy enough. Any the first step is to get the floor and see what the problem is.
Metal recyclers may be another place to get Aluminum at a decent price again usually by the pound.
http://www.yelp.com/biz/blue-collar-supply-sacramento