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.