First off…it’s Mardi Gras. [;)] In May? Hmmmm…hard to find Mardi Gras stuff three months after the event…
For King Cakes, try the CajunGrocer http://www.cajungrocer.com/default.php?cPath=1_15_26 . Actually, you can get pretty much anything you need there, now that I think about it. But wait…on reading the entire page, King Cakes are only available through March. Like I said…kinda hard to find real Mardi Gras items well after the event. I do have a recipe somewhere I can try to find if you’re interested.
Decorations? Think purple, green, and yellow. Crepe paper, beads, masks, feathers, etc. Aunt Sally’s http://www.auntsallys.com/mercmg.html has a bunch of stuff you could use, if you’re interested.