The Nios II EDS full edition contains the MicroC/OS-II source files as well as several example designs that use the RTOS and LWIP stack. The evaluation edition (freely available from www.altera.com/niosdownload (http://www.altera.com/niosdownload)) doesn't contain these files, but is otherwise a complete development platform. If you have a full Nios II core license, the evaluation edition can even be used to create shipping products.
The Nios II core license can be purchased from Altera or its distributors for $495. The ordering code is: IP-NIOS.