from where can i download el capitan image other than mac app store?
There's nowhere other than the Mac App Store where you can trust an installer to not give you a virus. If you have any place near your college with free wifi, going there to download El Capitan would be your best bet.

Alternatively, if you know anyone off campus with a Mac, you can have them download the installer from the App Store and then get it to you by giving you a flash drive or by using an online service like Copy (a Dropbox-like service that gives you 15GB for free).