You cannot get Leopard from Apple any more. You will find copies from third-party vendors on eBay, Amazon, or Google.
If you have an Intel Mac you can upgrade to Snow Leopard instead provided your model meets the requirements shown below:
Upgrading to Snow Leopard
You must purchase Snow Leopard through the Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.
After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store. Access to the App Store enables you to download Mountain Lion if your computer meets the requirements.
Snow Leopard General Requirements
1. Mac computer with an Intel processor
2. 1GB of memory
3. 5GB of available disk space
4. DVD drive for installation
5. Some features require a compatible Internet service provider;
fees may apply.
6. Some features require Apple’s MobileMe service; fees and
If you have a PowerPC G4 or G5 Mac, purchase Mac OS X 10.5 from a site such as eBay or Amazon.
If you have an Intel Mac, buy Mac OS X 10.6 from the online Apple Store.
In either case, back up your data first.