Apple no longer sells Leopard. Perhaps you should think of upgrading at least to Snow Leopard:
Upgrading to Snow Leopard
You can 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 iCloud services; fees and
You can upgrade from 10.4 straight to 10.6 with no problems as long as you have at least 1gb of RAM. Any program that runs under 10.4 should run under10.6.
See this list for third party programs compatibility with 10.6: http://snowleopard.wikidot.com/
You might have to upgrade some drivers for printers, etc.... And you will have to install Rosetta if you have any Power PC applications
To see if you have any Power PC programs go to the Apple in the upper left corner and select About This Mac, then click on More Info. When System Profiler comes up select Applications under Software. Then look under Kind to see if any of your applications are listed as Power PC. Universal and Intel will run under Lion.
Before Mac switched to Intel processors in 2006 they used Power PC processors from 1994 to 2005. Power PC 601 through 604, G3, G4 and G5. Applications written for the Power PC processors need the application called Rosetta to run on Intel processors. This was part of the Operating System in 10.4 and 10.5 but was an optional install in 10.6. With 10.7 Lion Apple dropped all support for Power PC applications.
I recently upgraded an older MacBook (Early 2006 model 1,1) straight from 10.4 to 10.6.
A retail 10.5 Leopard disk will run you over $200