For cloning, I recommend putting one of the computers into, so it essentially acts as a FireWire disk then boot the other computer from a third volume (your MBP's 10.6 system should work), run Disk Utility, and use its Restore feature to copy the 10.5.8 volume onto a blank partition in the MBP (be sure the enable the Erase destination option). I'm pretty sure the drivers etc for that model got rolled into the standard build of 10.5.8, so it should work properly. Another possibility would be to borrow an older Mac that can boot 10.5.0, install the base OS and update it to 10.5.8, and then clone the volume to your MBP.
Also, note that you'll need disks specifically for your model, as there was a special build of 10.5.7 (build 9J3050) with support for the new hardware in the Mid-2009 MBPs.
I'm not sure about Apple's current policies, but you might be able to order replacement DVDs via an Apple Authorized Service Provider or Apple Store - if so, they'd be treated a bit like repair parts, not as retail items.
