A is a great letter for a costume party theme. You can be as obscure or obvious as you like.
If you come up with any other ideas please feel free to add them to the comments.
Visit our costume catalogue to take a look at some pictures.
Abbess - Female superior of a convent / Nun
Abbott - Head of a monastery / Monk
Academic - Gown and mortar board
Ace - Pilot or aviator
Ali Ba Ba
Alice in Wonderland
Ambassador - any national dress to represent a particular country
Amphibian - frog
Ancestor - any period costume including cave people or ancient Romans or Egyptians
Ancient - Egyptians or Romans
Anglo Saxon - Vikings or Medieval
Archer - Robin Hood