http://www.anime-expo.org/

Anime Expo, Los Angeles is considered as the biggest Anime Convention in America.

The convention is traditionally held on the july 4th weekend.

The events which garner most attention at the Expo are Cosplay contest, Anime Music Video competition and the Charity Auction.
The exhibit hall at expo showcases products from various exhibitors and also the fan art. Apart from that other cosplay material can also be purchased.

Yes, of course there is also cosplay.