I currently have Studio One 2 Artist 2.6.5. Can I still get the latest version of Studio One 2 Pro? If yes, How?

No, we are no longer selling upgrades to Studio One 2 Pro.

With Studio One 3, you have some options if you do not wish to move to the Pro level.

You can update to Studio One 3 Artist and then purchase the add-ons that meet your needs.

Unless of course you need the Mastering Suite, then yes upgrading to Studio One 3 Pro is the only option moving forward.