OS X 10.5 Leopard is over 5 years old, now.
If you could still buy the retail disc from Apple, they sold it for $129 U.S.
It can still be found, but is getting harder and scarcer to find.
You could just try calling Apple direct at 1 800 MYAPPLE ask for retail software sales and see if they might still have OS X 10.5 Leopard discs lurking around. You'll still pay Apple's original price for the disc, though.
In all likelihood, you're going to pay more than original retail price for the official retail copy ( which you need) whoever has this for sale.
If you have access to Amazon.com, you can try there, too!