Go to the main Adobe website for your country. Click the buy link and
then search for InDesign CS6.
Keep in mind that you will be buying an already out dated version of
InDesign. The US price is $700.
Since Indesign CC is not the successor if Indesign CS6, because you have a totally different software handling and licensing system here which you can't compare, the CS6 is not outdated. CS6 is also the current suite you can buy at your local software store or shops like Amazon.
Your logic is completely lost on me. InDesign CS6 has no sure future beyond current operating systems and given the speed at which at which that's changing I wouldn't bet the mortgage on how long that will be.
If you want or insist on a perpetual license then yes, CS6 is the only choice, but it should be clear that subscriptions are the only way Adobe will release new software in the future.