Go with the classic french maid costume. This is going to get a lot of attention because it is so popular amongst the majority ot the population, forget anything else
If you want to be original - and not a little shocking, try dressing up as a guillotine! No realistic sharp blade necessary. Or you could be a baguette. Or one of the current president's model girlfriends. Good luck!
There's the classic striped black-and-white shirt, beret, red scarf, and black pants. Carry a baguette and say "Hon-hon-hon" a lot. Avoid interacting with actual French people.
Other possibilities - mime, French maid, 18th-century aristocrat (like Louis XIV or Marie Antoinette.) This last option could be quite lovely, although you'd want to rent a costume.
For traditional costumes, think Napoleon or Marie Antoinette. Going for laughs? Think anything with the word 'french' in it (ie. french fry, french toast, french braid, french kiss...)
1. French maid outfit.
2. Go to the local Goodwill and look for medieval costumes such as those used in plays. You can be a peasant or nobleman either way.
Fashion magazines!! There are many online ones you can refer to, such as Elle. For historical costumes, go to the wikipedia or browse in your local public library.