You want the Snow Leopard OS Disc, but I don't know about availability now since Lion has been out for a while. It was only $29 USD when sold in Apple stores and online. You can call your nearest Apple store, check out apple.com and also amazon.com. Last resort would be e-Bay but those are gonna be the most expensive. Supply/demand, etc.
What Mac do you have which would tell us if you are able to upgrade to Snow Leopard or not..and MYmacROX gave you ideas on where to get the SL disc from assuming you can upgrade to it..
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.