How can i have snow leopard again without CD?

Hello! I had to format (sorry i know this is not the term, but english is not my mother language) my late 2007 MacBook(the white one) which had Snow Leopard 10.6.8 that i downloaded for free back in the days and now i have Leopard 10.5.8 that i have put with the CD. How can i have snow leopard again provided that i don't have the installation CD?? Is there any chance that Apple will send me the update for free giving them my serial number? If Yes what should i do?? Thank you in advance