This is what you will need to officiate:

2 shirts (new version) yellow and green (minimum, red would be a good 3rd option).
1 referee shorts
1 two-stripe referee socks (Not 3 stripe)
Whistle (Preferably a Fox 40)
Watch and/or stopwatch
Yellow and red cards
Score sheet wallet (Official Sports version is best as it fits in the pocket)
Flag set
Velcro backing for badge
Bag to put it all in (optional)
Black undershirt (optional)

Where to buy?
Official Sports International
Officialsports.com
Go to Starter Kits, then check out W1815 11-piece set for women and 1815 for men.