Adobe no longer supports 10.5 Leopard. You will have to upgrade to 10.6 to use the latest flash.10.5 Leopard was based on Power PC code and was written to be able to run on both Power PC and Intel processors. 10.6 Snow Leopard has completely rewritten code and only runs on Intel Macs. So anything written to run on 10.5 has twice the code needed for one written for 10.6 or newer.
The 10.6 Snow Leopard DVD is back in the Apple online store. You can get it for $19.99. You will need to be running 10.6.8 to access the App Store to order Lion or Mountain Lion. http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
It may have been disabled by a recent security update.
Get a fresh copy from here; http://get.adobe.com/flashplayer/otherversions/
Put your operating system in the first drop-down, click the second one and you'll be offered v 10.3 to download.