from here ${URI_BASE}${URI_BASE_NAME}.tgz - it's in src_unpack (which I know is not really a good place to put this) but it's a versioned  tarball for which they do not grant direct access to their repository (where they use CVS). 

The sources are mirrored under git in a debian repo ( git://git.debian.org/git/pkg-exppsy/afni.git , as detailed here http://afni.nimh.nih.gov/afni/community/board/read.php?1,142446,142566#msg-142566 ). But I do not know in how far it would be preferable to pull from debian rather than from the NIH.