It would be cheaper to get an external HDD, install
Mac OS X 10.6 Snow Leopard onto that, create a second partition roughly the size of the internal MBP's HDD, use
CarbonCopyCloner (version 3.4.7 is still free) to clone that internal HDD to the second partition, install
Mac OS X 10.6 Snow Leopard via a Clean Install onto the internal HDD and use Migration Assistant to migrate your data and applications from the external HDD.
Or even simpler just install
Mac OS X 10.6 Snow Leopard onto the external HDD, migrate from the internal HDD if possible and then clone that back to the freshly formatted internal HDD.
In any way, you should have a backup, especially for software you have paid for and do not have the install media anymore.
And even if you have not paid for the software and do not want to acquire it again for whatever reason, a backup is a necessary tool for many cases. HDDs WILL fail, some sooner, some later. And a backup is cheaper than data recovery (software for 100 USD, using a company and sending your HDD in 1,000 USD and more).
It is your computer and your data, act accordingly.