Where to buy a bulk quantity of Three mobile broadband Sim cards?

I noticed on Amazon and Ebay that people offer Three Mobile Broadband Sim cards. I often get these and pass them on to other people. But does anyone know where these sellers get their stock from? I have contacted Three myself but they say that they do not supply these sellers! So I would like to know how these third party sellers get hold of this stock, which is genuine and original… Any help and advice would be most welcome! Thank you