Your best bet is to look on eBay for a copy or ask a friend for his old copy.
Personal note: I'd suggest you work towards getting an Intel based machine as the PowerPC machines are getting old and may have some problems running new software.
You will want to find a Leopard Retail DVD. The discs are black. Grey discs are specific to the machines they came with so none of them will work on your G4. Just make sure that your G4 is compatible first.
Just keep an eye out for them on ebay. You can find them for under $50 once in a while.