You need to use the install DVD which you used to install the previous OS or which originally came with the computer; if you don't have the computer's original disks and it shipped with 10.6.8 or earlier, phone Apple and order replacements.
No downloads of 10.5 or 10.6 are available to the general public, free or paid.
You cannot download OS X. If you have Snow Leopard you can reinstall it.
Downgrade Lion/Mountain Lion to Snow Leopard
1. Boot from your Snow Leopard Installer Disc. After the installer
loads select your language and click on the Continue
button. When the menu bar appears select Disk Utility from the
2. After DU loads select your hard drive (this is the entry with the
mfgr.'s ID and size) from the left side list. Note the SMART status
of the drive in DU's status area. If it does not say "Verified" then
the drive is failing or has failed and will need replacing. SMART
info will not be reported on external drives. Otherwise, click on
the Partition tab in the DU main window.
3. 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.
4. Quit DU and return to the installer. Install Snow Leopard.
This will erase the whole drive so be sure to backup your files if you don't have a backup already. If you have performed a TM backup using Lion be aware that you cannot restore from that backup in Snow Leopard (see below.) I suggest you make a separate backup using Carbon Copy Cloner.
If you have Snow Leopard Time Machine backups, do a full system restore per #14 in Time Machine - Frequently Asked Questions. If you have subsequent backups from Lion, you can restore newer items selectively, via the "Star Wars" display, per #15 there, but be careful; some Snow Leopard apps may not work with the Lion files.
What OS was installed prior to Mountain Lion? It must have been Snow Leopard, for which you must have an installation DVD, correct? You can use it to install Snow Leopard again, but doing so will completely erase your Mac. Back it up using "clone" software or Time Machine, if you have any data you wish to keep.
There are no legitimate sources from which to download Leopard, and retail copies of it can be prohibitively expensive.