where can I buy or download Pages for mac os sierra 10.12.6?

The only Pages that you can buy will be the retail, (not the trial) iWork '09 DVD from Amazon or Ebay resellers. After that installation, you would update it with the Apple iWork 9.3 updater. Apple retired that product in Sept 2013, and this version of Pages ceased to function after the Mojave 10.14 upgrade. The Pages '09 application cannot open Pages documents created by later releases of Pages.

Apple does not keep full installers of past Pages products available for purchase or download. The current version of Pages is v7.3, and it requires High Sierra 10.13 or later as a prerequisite for installation.

The retail iWork '09 applications are only available on DVD, and if you find it in a software download form, then it is trialware that will expire without any means to purchase it now, and/or it contains hidden malware. Just don't go there.

Your 2013 MacBook Pro is capable of upgrading to High Sierra (10.13.6), or Mojave (10.14.2). Either upgrade would satisfy the installation requirements for the current, free Pages, Numbers, and Keynote applications in the Mac App Store. These are ongoing, supported applications. Those linked, hardware specifications will also show what your MacBook Air can upgrade too.

The link to the free, High Sierra 10.13.6 upgrade. The link to the free Mojave 10.14.2 (as of this post) upgrade.

If you plan to upgrade the operating system, invest in an external USB3 drive that is at least three times the size of your internal boot drive. Do not install any third-party drive vendor's software — it is unnecessary, and will eventually cause problems. Use Time Machine to perform a full backup to that external drive before attempting any operating system upgrade.