You can always start from
https://www.kernel.org/ .
Current path is
https://www.kernel.org/pub/linux/kernel/projects/rt/ .
Yes, the last kernel is
https://www.kernel.org/pub/linux/kernel/projects/rt/2.6.33/ , IMO not
unusual that the patch is behind current vanilla kernel version.
I can't speak for NFS, but at least apart from that 3.6 kernels seem to
be ok. If you need NFS and it should be buggy, can't you use an older
version or the current kernel full preempted with threadirqs? I still
prefer the Kernel-rt, but to be honest, a full preempt kernel with
threadirqs, if any, just makes a minimal difference.
YMMV,
Ralf