Hey there Barbido,
I apologize for the late response, and thank you for your patience! Currently, there isn't a way to purchase a Chromecast power supply separately. In this case, you'd want to talk to Chromecast Support. If you don't mind, would you please fill out this form to have Chromecast Support reach out to you?
I look forward to your response, and if you have any other questions or concerns for me, please let me know.