It may be difficult to find Snow Leopard (10.6) in a retail store as Lion as been the current OS for nearly a year now. You can still purchase it directly from Apple:
You'll want to make sure your system meets the minimum requirements found here:
The "minimum" requirement is 1GB of RAM, but you really need to have at least 2GB for it to be usable and preferably 4GB for smooth operations.
All Intel Macs, so long as they meet the requirements in the link above, are compatible with Snow Leopard (10.6). There is no download for Snow Leopard. You must purchase the actual DVD. After installing Snow Leopard, you will be able to run Software Update to get all the updates up to 10.6.8. You will need to perform the updates in order to get access to the App Store.