Depending on your location, find a server near you from this list:
http://www.freebsd.org/handbook/mirrors-ftp.html
Not all of the servers have all the ISO-images, check in
/pub/FreeBSD/releases/i386/ISO-IMAGES/ (or alpha/ISO-IMAGES..) and if you
can't find the .ISO you're looking for, try another nearby server.
(The one I'm using is having 3.5.1, 4.1.1, 4.2 and 4.3b2,
ftp://ftp.se.freebsd.org/pub/FreeBSD/releases/i386/ISO-IMAGES/ )
--
Frode Gill