10.4.11 to 10.5
10.5 (Leopard) is a paid upgrade can must be purchased.

I don't believe Apple actually sell it anymore so your choices are eBay or places like that.

You will probably have to pay above the original price for it.
Amazon has it from $139, upwards.
eBay for $119 and up.

BEWARE: Do not purchase any DVD that is not described as RETAIL. The Restore disks are machine specific and may not work on your iMac.

P