Where can I get install discs for OS X 10.5 to max out a Powerbook G4? Over the past year, my dad's Powerbook G4 (OS X 10.3) lost the ability to run any usable version of firefox or other end of life PPC software. Apple customer service wouldn't sell him a copy of 10.5 and told him to search ebay for copies of OS X 10.5, 8 months after releasing 10.6. We've been watching ebay and the price just keeps climbing above the original price.

I told him to buy a copy of 10.6 and the license should allow for installing older versions. He took it to a 3rd party Mac dealer and they said they couldn't install 10.5 if he doesn't own a 10.5 license (which no one will sell anymore). Does the OS 10.6 license cover older versions of OS X? Can 10.5 be obtained for the original price anywhere?
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.