Yes, you can. Buy Snow Leopard here: http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
You could even update to Lion (10.7), if you wanted to (you need Snow Leopard, and update it to 10.6.8 from 10.6.3 first). You would need to call Apple to order it.
But, if Snow Leopard meets your needs, you don't have to.