If the system is capable of running 10.5, you'll find it cheaper to buy 10.5 than get an old copy of 10.4.
FYI, all retail copies of 10.5 will work on PPC or Intel systems (assuming the Intel system isn't so new that it needs the original included discs), while retail copies of 10.4 ONLY work on PPC. If you do buy a copy of 10.4, you should check to see what month/year your PPC Mac was released. Make sure the retail copy of 10.4 is a version AFTER that date. 10.4 was produced in retail for 10.4.0, 10.4.3, and 10.4.6. For instance, if you have a Quad G5, you need original install discs, 10.4.3 retail, or 10.4.6 retail to boot it.