You can get Das U-Boot source from DENX, http://www.denx.de/wiki/U-Boot/WebHome
. The kernel source comes from kernel.org. The links I listed earlier are from the Debian for ARM project, http://www.debian.org/ports/arm/
But don't ask me how to compile a kernel. That would be new ground for me. I plan to install developer tools and sources on my SheevaPlug on a working MMC/SD Debian setup. If I have any success I will post on this forum.