I've could not find 7za in any yum repositories on RHEL/CentOS, so I've just downloaded the latest version of the source from sourceforge.net:
$ wget https://sourceforge.net/projects/p7zip/files/p7zip/16.02/p7zip_16.02_src_all.tar.bz2
$ wget https://sourceforge.net/projects/p7zip/files/p7zip/16.02/p7zip_16.02_src_all.tar.bz2
(change "16.02" above to whatever latest version atthe time when you try to download, you can find out by going to https://sourceforge.net/projects/p7zip/files/p7zip/ ).
Being in the directory where you downloaded the p7zip archive, unpack it:
$ tar -xvjf p7zip_16.02_src_all.tar.bz2
$ tar -xvjf p7zip_16.02_src_all.tar.bz2
Then run the make command as root user inside of extracted folder:
# make
# make install
# make
# make install
It'll produce the 7za
binary in the /usr/local/bin
directory (if you use RHEL7 at least):
$ whereis 7za
7za: /usr/local/bin/7za
$ whereis 7za
7za: /usr/local/bin/7za