You can't get it directly. You have to order the thick aluminum "U" shape channel stock and then use a key cutter tool on a milling machine to cut the groove for the bolt heads.
The other method is to make it all out of aluminum bar stock. You cut the groove and then cut the bolt recess for the Tee shape.
Of course there may be a machine or something out there that uses it already, where one could possibly order the part, but I never ran across it myself yet.