Your 15" 1.5GHz PowerBook probably came with OSX 10.3.x Panther. Officially, it could run up through OSX 10.5.8 Leopard, but in my experience, the entire system will run best with OSX 10.4.x Tiger.
As stated above, you can make a bootable disc to run Linux. This would probably be your lowest cost option.
As for installing Mac OSX, you'll need to find a set of OEM Restore discs for your era of PowerBook G4. You'll need to figure out which specific PowerBook G4 you have as the Restore discs are very picky. Try looking online based on your computer's serial number. The restore discs are available on eBay, but you'll probably pay somewhere around $100+ for 'em.