Pick a favourite character and make your own using what you can find, a few fun ideas could be: Marilyn Monroe, one of the Simpsons, wonderwoman/cat woman/a super hero, a pink lady from grease, Jessie the cowgirl from toystory, or just a pick an era and base your outfit around that, like 20s flapper girl or 70s rock chick or 60s flower power.
I think Flowergirl Dresses would be the best for you.As you said you want to wear this on your birthday eve.In this dress you will look like a bridegroom.As we are clothing specialist and we know about modern fashion.
any costume can be cheap (apart from cosplays) just if you put more money into it it looks better but popular and easy choices are normally cats, pirates and angels
I'm not sure what you mean by wanting people to not know who you are. Do you mean that you want to wear a costume? I suggest you order something on eBay or a chinese online store. Or like a fancy dress that is a bit garish? Thrift stores will be your best friend in this scenario. Or you just want to look glamorous and shock everyone? Then any old store will do. Try looking at places that do prom dresses and I'm sure you'll find a dress or two. Happy 18th!
Forever 21 has an amazing and affordable selection of dresses for any occasion :)
Hollister has good dresses.