How do I buy a core?

Excuse me, could you possibly enlighten me on a subject I'm somewhat puzzled about? I've been hearing a lot about the purchase of a 'core' recently, but I'm not quite sure how to proceed. Could you possibly explain the steps involved in acquiring one? Is it something I can purchase online? Or do I need to go through a specific platform or exchange? Additionally, are there any regulations or restrictions I should be aware of? I'd greatly appreciate your guidance in this matter as I'm quite eager to embark on this journey but am also cautious not to make any mistakes. Thank you in advance for your assistance.