The only way you can receive an HeroPoints code is to purchase a HeroMart product that comes with HeroPoints. Here is where you can find what HeroMart products come with HeroPoints.
In-game items are not sold outside of the games, HeroPoints act as a bonus for your the specific HeroMart product(s) you choose to purchase. The HeroMart and the Artix Games staff work very hard to create the HeroMart products and in game items for you to enjoy!
AQWorlds does offer calendar class variants you can purchase with AdventureCoins! Here is the Design Notes post for more information.