A cross compiler is a compiler set up to run on one ISA but generate binaries which run on another. You may need one if you intend to simulate a system which uses a particular ISA, Alpha for instance, but don’t have access to actual Alpha hardware.
There are various sources for cross compilers. The following are some of them.
Alternatively, you can use QEMU and a disk image to run the desired ISA in emulation. To create more recent disk images, see this page. The following is a youtube video of working with image files using qemu on Ubuntu 12.04 64bit.