Craigslist or eBay, but you have to have luck. But beware of the grey Restore DVDs, you probably want a Retail DVD I guess. Do you have a PPC or Intel Mac?
A new set will set you back some moolah, and be wary of people selling a grey dvd that came with their laptop or mini but not the real "good for all compatible Mac installs" Leopard dvd. I needed a copy last summer for PPC, was fortunate to find a NIB set with iLife '08 on Amazon whom I consider trustworthy, paid $225.