Neither Leopard now Snow Leopard are available for download.
Leopard is only available on DVD from sources like Amazon or eBay.
Snow Leopard can still be purchased from Apple:
Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
http://support.apple.com/kb/SP575
(NB: PowerPC applications can still be run in Snow Leopard using Rosetta, but they will not work in later versions of OS X. For detailed information see this user tip: https://discussions.apple.com/docs/DOC-6271 )
The OS 10.6 Snow Leopard install DVD is still available for $19.99 from the Apple Store:
http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
and in the UK:
http://store.apple.com/uk/product/MC573/mac-os-x-106-snow-leopard
Before installation you should back up all your data.
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard to bring it up to 10.6.8, or download the combo update from here:
http://support.apple.com/kb/DL1399
Check via Software Update whether any further updates are required, particularly to iTunes (you will need the latest version in order to synch with mobile devices).
