You can buy Leopard 10.5 on eBay. If your computer doesn't meet the system requirements, Tiger 10.4 would be a good choice. These discs are in high demand and you could pay anywhere from $20 to $150 depending on version and condition (10.4 completed listings here and 10.5 completed listings here. The 10.5 discs are particularly expensive and generally sell for $100 or higher. iMac G5 1.8GHz, 2GB RAM, 160GB, Mac OS X Leopard 10.5.8, SuperDrive
PowerBook G4 1.33GHz, 1.25GB RAM, 60GB, Mac OS X Tiger 10.4.11, SuperDrive
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