There are actually many ideas to planning a masquerade ball. It all depends on how elaborate you want the event to be. Obviously, you'd want to know where to get masks for each of your guests. So, here is an initial list of findings from making them yourself to buying them ready-made. Some are pretty pricey, since they're made by artisans in Venice. You can also check out places that make costumes, and eBay is also a great place to find masks.
Mardi Gras Outlet
Naughty Magpie
Mask Italia
Visions of Venice
DIY
From BBC - how to make a Venetian mask
Apollo's Fire (the Cleveland Baroque Orchestra)
DIY Network - Paper Sculpture Masks
For some educational reads:
Venice Carnival - official site featuring travel, hotel, and event information.
History of the Carnival of Venice
