Look up and see the menu called Gear.
There are about a thousand different vests, backpacks and other pouches you could buy, and a billion different ways to put them together. Check out
http://www.militarymorons.com/ specifically the 1st and 2nd line gear pages, and our own Groombug's RAV guide:
http://www.airsoftcanada.com/showthread.php?t=26622