Is it posssible to add default repository/repositories to SLES when the original repository is unavailable?

The "default" repositories of suse are listed here. I guess you might at least want to add Official Repositories > OSS.

zypper addrepo http://download.opensuse.org/distribution/11.4/repo/oss/ oss

For SLES, you simply register the server:

suse_register -a regcode-sles=yourRegistrationCode -a [email protected] -L /root/.suse_register.log

Then, the repos will be added automatically. You can also log into your account here and click on Base and Extension Products. I will give you one of the URLs I see there, but it will be useless to you because the repos require authentication before you can connect and also because I'm using the Itanium version. Here is an example repo:

https://updates.suse.com/repo/$RCE/SLES11-SP4-Pool/sle-11-ia64

The repos listed in the other answer are not the official SLES repos.