Old-fashioned masquerades (not costume parties) can be great fun. Traditionally, the host supplies the masks, and that's a task that your party rental supplier can help you with. They'll also be able to offer tips and other supplies to make your masquerade go down right.
When contacting a party rental business, don't be afraid to ask for exactly what you have in mind. You might be amazed at the sheer variety of tents, shelters, chairs, linens, glassware, and specialty items that a really good party rental facility has to offer.