[mothered]

Buy them

Where?

No, you look for one that is selling on ebay. You look at the pictures of the card that have been posted, and get the serial number there.

This ^

It can be difficult at times to locate an Image with the serial displayed. In such cases, SE the seller by simply asking for It, for the purpose of verifying It's validity prior to purchasing. Of course, you have no Intention to buy It.