Aptitude doesn't find nginx

I just made a clean install of 9.04, and then wanted to install nginx etc. Aptitude can't find the package though (and also a few others, like mercurial). Do I have to add additional repositories for this? If yes, how can I find out which ones I need? I don't see anything here that would indicate that nginx is not part of the standard Jaunty repositories. Any help would be appreciated.