Found an interesting thread on the subject that might be worth a read:
http://forums.cosplay.com/showthread.php?p=843483)
I know whether you buy it or make it, it's very expensive, so I guess you better really want it! Too bad they don't just rent the stuff (though cleaning it between customers might be a pain).
You'd think
http://www.501st.com/) would be a place to start, but I didn't find any useful information on there on first glance. I guess they pretty much just assume you already have your costume before you join... but I could be wrong.