The most common kinds of aluminum extrusion I have seen in FIRST Robotics, is 80/20 and Bosch.
80/20 is I think what you are looking for. (The description of "Plus shaped" kinda gave it away)
See attachment below.
That size is the 1010 series.. and is approximately 1" x 1" square(ish) lol.
http://www.8020.net/