http://www.altera.com/products/ip/ipm-index.html (http://www.altera.com/products/ip/ipm-index.html)
That's if you were looking for IP in general (under downloads). For NIOS click on the "Embbeded" link
Some of those cores are free, some are time limited (they time-out after a while), and some you will have to request from the company that supplied it (and they may expect licensing fees if you use their core).
Cheers
