This item is temporarily unavailable Mac error

After my macbook pro had become really slow the past days, I decided to reset the system, following the usual steps:

1. Press alt+power button

2. Choose Utilities --> erase Macintosh HD (or in my case, it was called macbook pro)

3. Reinstall the Mountain Lion OS X through internet.

However, after trying to install the OS X, and entering an apple id (my brother's, since his id worked - probably because he used his id when installing the OS X on this computer the first time), an error message pops up, saying:

"This item is temporarily unavailable. Please try again later"

I don't know what to do, since I have tried redoing these steps on multiple occasions this day, and with two different networks. Unfortunately, I don't have the original installation dvd:s, so I have to install the OS through internet (or if it is possible, by downloading the OS to an external hard drive, and then installing through that).

Anyways, my macbook pro is bought in the mid 2010's, maybe that information can help? I don't know what the last OS X update I used was...

[Re-Titled by Host]