where to grab source tarball?

[John Almberg]

I'm trying to upgrade FreeBSD from source, but my /usr/src directory  
is empty. "Absolute FreeBSD" glibly says to "grab the source tarball  
from a FreeBSD mirror".

I found a list of mirrors here:

http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/mirrors- 
ftp.html

But it isn't clear to me which tarball I need to 'grab', or where it  
is on the mirror.

Basically, I want to get to the point where I can type:

cd /usr/src
make buildworld

And build FreeBSD 7.1-RELEASE

I'd like to download the source to the server, rather than inserting  
a CD in the machine, since I"m 2 hours away from the machine.

Thanks: John