You could ask the person who sold you the MacBook if they still have the discs.
If they're not there anymore: If they sold the license to you, you can basically download a 10.6 installer "somewhere else" on the Internet and use that. (The actual installation medium isn't tied to the license—anyone can lose or scratch their discs.)
In order to reinstall OS X you'd have to either burn the disk images to a DVD, or restore them onto a drive which you can then boot from USB. Note that in order to partition the system you have to start Disk Utility from the setup after you've booted from the installation medium.
I have some programmes (CS2) that I think will run on 10.5 but probably not 10.6
I don't understand why you'd want to reinstall 10.6 then and not go back to 10.5. Have you tried the programs? If you're already on 10.6 you can install them and see if they work.