The general assumption is that Apple no longer has OS 10.5 install disks. However, some people in the US have been able to get a copy by phoning the US Store at 1-800-MY-APPLE and asking for an operating system specialist, than asking for OS 10.5 install disks. Not a "sure thing" and the last ASC report of someone doing this that I've see was about 16 month ago. Still, it costs nothing to hear the word, "no," but you might get lucky.
Otherwise you are thrown into a miasma of confusing third-party offerings. Legitimate copies of old Mac OS software, especially 10.4 and 10.5 are in high demand and, as such, attract the scammers and the scalpers. I have seen legit copies of 10.5 offered for more than the market value of a used G5 iMac. Not worth it.
Try auction sites or Amazom but remember there are NO SAFE, LEGAL downloads of any Mac OS software. Some people will sell you a download license that doesn't exist. Another resource is a Mac Users group. Find one in your ara with this link:
http://appleusergroupresources.com/?s=apple+user+group+near+me
Gray disks from another Mac model will likely not work, and a gray disk that shipped with an intel Mac definitely won't. A full retail install Leopard 10.5 disk looks like this:
