PowerBook G4 2005 won't boot past a blue screen.

Apple have released no legitimate OS 10.5 disk images for download. Anything offered as a download is pirated. Your only option is a disk and, as I see, you are aware of the issues with finding a legitimate one on the open market.

Given your concerns, I feel your best option is to work through a Mac Users Group (MUG) in hopes someone has a proper 10.5 retail install disk for sale. You can use this link to search for MUGs in your area:

http://appleusergroupresources.com/find-a-group/

RE: NY-MUGs

they all have email addresses and most (All?) have websites - I would start there rather than 'showing up at a meeting' anyway.

Allan can advise best on optical disks available on eBay... I would imagine that you would need a "Retail" or "came with the Mac" version

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I've looked at alleged "full retail install" Mac OSX disks on eBay for years and do not have a lot of confidence in your finding a real disk there. Their are too many that are the gray disks that are quite hard to match to your computer sub-model or, on further reading, one finds the advert is for a downloaded version--the automatic red pirate flag goes up.

As CCC suggests, you can e-mail several MUGs with a subject title such as "Looking for.." and see if they respond.

The gray disk issue is big. Apple changed disks within models, such as between a Rev A and Rev B. There is no easy chart to look up the proper Apple part number for the gray disk that is compatible with your computer. They are specific to the hardware profile of the computer with which they shipped, so likely won't work on a slightly different PowerBook. They should be avoided unless you plan to buy a couple of dozen, hoping you find one that works.