According to the reports of Techcrunch, old MAC users who have not previously purchased the software can have redemption codes on its online store for both OS X 10.7 Lion and OS X 10.8 Mountain Lion.
Both 10.7 Lion and 10.8 Mountain Lion costs $19.99 each, and after purchase user will get an email with a content code which could be redeemed within the Mac App Store. The most updated version of OS X is OS X 10.9 Mavericks.
Earlier, OS X Mountain Lion was available via the Mac App Store and to purchase a copy of it was only possible after a call to Apple. This system is removed to promote new Mavericks, and this was the only way to acquire an older Apple OS.