alpha-sources don't exist in the tree, but the doc says "alpha-sources, [...] are, as their names suggest, patched to run best on specific architectures. They also contain some of the patches for hardware and features support from the other patch sets mentioned above and below." which suggests that they can be installed using portage (this paragraph is located in the section "supported kernel packages").
Thanks for bring this to our attention. I'll fix it up.
Fixed in CVS, thanks.