how can i get jet set radio because he is out of steam (I also accept suggestions for "another" websites if you understand me, before I get the original game)

The game is still available on PlayStation 3, which you can get the PS3 version and/or the PlayStation Vita version. Unfortunately, that is the only place to purchase currently. This also may not be the case for every region. It is currently still available on the USA PS3 marketplace. Otherwise you can get a used physical copy for the Dreamcast.

As for Steam, you better wish luck that your friend can gift you their copy. Other than that, you're on your own when it comes to PC.