How to buy polygons on MetaMask?

I'm curious about the process of purchasing Polygons on MetaMask. Could you walk me through the steps in a clear and concise manner? What are the prerequisites I need to have in place before I can make a purchase? Also, are there any fees associated with the transaction, and if so, how are they calculated? Lastly, is there a limit to the amount of Polygons I can buy at once, or can I purchase as many as I want? I'd appreciate your detailed response.