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. 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.
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. 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. For cloning, I recommend putting one of the computers into target disk mode, 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).
In case of trouble, once you have the system installed you should probably make a disk image so you can restore it if anything goes wrong (you're running tests, right?). Again, while booted from another volume (e.g. 10.6), run disk utility, choose File > New > Disk Image from Folder (don't use the from device option), select the entire bootable volume as the source "folder", save the image as either compressed or readonly on some volume with plenty of free space (compressed takes a lot longer to create, but the space saving is generally worth it). Once that finishes, choose Images > Scan Image for Restore, and select the newly created image (again, this takes lots of space as it actually copies the entire image in the process of "scanning" it). When that's finished (it takes quite a while -- everything in this process does), you have an image that can be used as a Restore source in Disk Utility.