If your computer is compatible with ML, http://www.apple.com/osx/specs you can download ML for $20. Then do the install.
However, to factory restore you have to the SL disk as the ML installation is not factory condition for your Mac. If your goal is to sell it, the ML license agreement says you cannot sell the Mac with your copy of ML installed as it is tied to your Apple ID.
If you need the SL install disk for your machine you can call Apple support and they will send a disk for a small fee, around $20.
Consider this, then:
Make Your Own Mountain/Lion Installer
1. After downloading Mountain/Lion you must first save the Install Mac OS X Mountain/
Lion application. After Mountain/Lion downloads DO NOT click on the Install button.
Go to your Applications folder and make a copy of the Mountain/Lion installer. Move
the copy into your Downloads folder. Now you can click on the Install button. You
must do this because the installer deletes itself automatically when it finishes
2. Get a USB flash drive that is at least 8 GBs. Prep this flash drive as follows:
a. Open Disk Utility in your Utilities folder.
b. After DU loads select your flash drive (this is the entry with the mfgr.'s ID and size) from the left
side list. Click on the Partition tab in the DU main window.
c. Under the Volume Scheme heading set the number of partitions from the drop down menu to one.
Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the
partition scheme to GUID then click on the OK button. Click on the Partition button and wait until
the process has completed.
d. Select the volume you just created (this is the sub-entry under the drive entry) from the left side
list. Click on the Erase tab in the DU main window.
e. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, check the
button for Zero Data and click on OK to return to the Erase window.
f. Click on the Erase button. The format process can take up to an hour depending upon the flash
3. Use Lion DiskMaker 2.0 to put your installer clone onto the USB flash drive.
Note: The term Mountain/Lion used above means Lion or Mountain Lion.
Once you have a bootable flash drive installer, boot from it then you can erase your internal hard drive and install Mountain Lion on a freshly formatted disk.