Where can I get install discs for OS X 10.5 to max out a Powerbook G4?
Firstly, I'm not sure where you heard that 10.6 shipped less than six months ago. It actually shipped in August 2009, making it over 12 months old.

Old system software tends to sell at a premium. I'm not sure why, but it may be at least partially because Apple no longer produces them. However, as Apple only sells 10.6 as a stand-alone OS (that is, it doesn't sell upgrade versions, just full) you can legitimately buy 10.5 from anyone with a copy who has since upgraded to 10.6.

There are plenty of places to buy 10.5 if you're willing to shop around. Keep an eye on this search and this search and see what turns up. For instance, this eBay sale is currently going for $40.