PropDad:
Is there a particular online site that is best to buy from?
I wouldn't say there is only one. However, I've had good experiences with products from both Adafruit and Sparkfun, and they both have starter kits (check the links). There's also the official and still fairly new Arduino Starter Kit, though you'd probably want to buy it from a North American distributor (e.g. Adafruit sells them).
By the way, did you get what you wanted accomplished on the costume in time for Halloween?