The main Samba site is nimbus.anu.edu.au in pub/tridge/samba/. This is 
maintained by Andrew.Tridgell@anu.edu.au

Mirror sites include:

ftp://src.doc.ic.ac.uk/packages/samba
ftp://ftp.demon.co.uk/pub/unix/samba
ftp://sunsite.unc.edu/pub/Linux/system/Network/Samba/
ftp://ftp.choc.apana.org.au/pub/samba

There are several others. Give archie a try.

SCO binaries available from:
ftp://ftp.markv.com/pub/samba (built by lance@fox.com)

Http sites include:

http://lake.canberra.edu.au/pub/samba
http://www.choc.apana.org.au/pub/samba