You can purchase the expansion bundle through the following official and third-party platforms: Guild Wars 2: Path of Fire FAQ
Buying (HoT) is no longer a standalone purchase. Since August 2019, ArenaNet has bundled it for free with the second expansion, Path of Fire (PoF). This means when you buy Path of Fire, you automatically unlock all Heart of Thorns content as well. Purchase Options & Pricing buy gw2 heart of thorns