I have a used Mac Book Pro and cant find a factory system restore disc...

You can't find any purchase for that kind of software. You have to take the computer to apple and they can restore the machine for a minimum charge.
All the purchase sites you can find are just virus or trojans so do not purchase those! You can destroy your computer for nothing!
Every single Mac Book Pro has differences, not even you find version 10.5.8 you need be alert about the inches on your mac and the year of this one.
To know exactly specifications of your Mac visit this website:
www.everymac.com
You only need fill on the search space the last 4 numbers of your serial and you can get all the specification including the year.
The best option its you try to find those dvd disks on ebay.com
If you find someone with disks similar to those you are looking first ask if are good for your model. If you get the wrong version never works.