Your description of how you got Mountain Lion installed doesn't make much sense. You need to download it from the Mac App store so that it is registered as purchased by the Mac App Store. If someone from an Apple Store installed it for a cash transaction, that seems rather odd.
However, open up the App Store, Purchases and see if it is listed there.
Actually my OS got corrupted, so I went to the Apple Service Center. They charged me some money for reinstalling the OS and they didn't gave me its dmg file. Also during installation they didn't even asked my apple id. Because of which this purchase is not showing in the App Store.
Now I want to download the dmg, so what should I do ? How can I download it now without paying it for the second time ?
As sig says, we can't do anything about what that Service Center did. You need to contact them.
There isn't a dmg file for them to give you. You download it from the App Store.
If it doesn't show in your Purchases tab, enter your Account and click the link to show hidden purchases. See if it shows up then.
What OS did it ship with? 10.6 (Snow Leopard), 10.7 (Lion) or 10.8 (Mountain Lion)?
If it shipped with Lion or Mountain Lion, you should be able to use Internet Recovery to reinstall, if needed: http://support.apple.com/kb/HT4718