If you find yourself bit, kill the spider and take it with you. You'll forestall any complaints and questions about whether you're sure it was a black widow and shorten the time to diagnose and begin treatment. (I.E. You'll experience far fewer of the symptoms the sooner you get treated.)
(Unfortunately as of a few days ago, there is still a storage of supplies for the antivenin: http://www.ashp.org/menu/DrugShortages/CurrentShortages/bull... So some hospitals may not have it in stock if they normally would. But apparently emergency supplies remain.)