First, Ubuntu is a dpkg-based Linux distribution, not rpm. So use
dpkg to install packages.
Second, MySQL 5.5 is much better than 5.1. And the latest Ubuntu release (precise) provides only 5.5 now.
Well, since you have older releases, you can still use 5.1. The package
mysql-server will always match the latest version of MySQL for your release.
Since you do not have internet, you can run
sudo apt-get install -sV mysql-server locally without connection. The option
-s means "dry run" and the option
-V means "show version number". This command does not require root permission and will not install anything really. It will just show list all the dependencies the package
mysql-server needs and simulates installing them. Thus you can get both the dependencies, the architectures and the version numbers. You can use a redirection to save them to a text file and copy it to a computer with Internet connection. With this list of packages, you can download them on the computer with Internet and copy them back to the computer without Internet.
On the computer without Internet, you can use
sudo dpkg -i *.deb to install all the .deb files you've downloaded from another computer.