You can buy them here: [link] they sell them for $4.25 per marker. The retail price is $6.49, so this website actually has them for a great deal. I buy mine at Hobby Lobby, but they might not have one in your area Copics are well worth the price though!
You used to be able to directly get them from the website, but now they just have a 'find a dealer in your area' thing. Some art stores will custom order things for you, you may wanna check at your favorite one. ^^