Purchase older version license

Hello,

For some reason (not technical), I need to install a PLESK 11.5.30 on a Centos 6.2.

Where can I buy a license for this version ?
I find only v12 licenses on http://www.plesk.com/plesk/buy-plesk/.

Or maybe, the v12 licenses are compatible with PLESK 11 ?

Thank you for your help.