I am in love with the new Sweetheart Darkonite and need to adopt one, but how do I get him?

you can only get one via Zapper or elixir, like

They are limited edition pets. You need to first use an elixir to turn it into some kind of darkonite, then the sweetheart potion to make it sweetheart.

In user shops elixirs range from 25,000 to 100,000 sP or so.

Thank you both for the information. I am gonna get started right away!