A lazer cutting shop will be the best bet
http://www.google.com/search?q=laser...ed=0CCkQtQMwAA
, as for the size I would not go over 1/4" as it will just be overkill personly 3/16" would do it for me also if you get it lazer cut it wont twist or bow out of flat, your only going to be supporting a few kilo at max right?