Buy users..... It doesn't sound very good. I recommend that you develop your accounts and users will come to you. It will be more useful.
It depends on what do you need it for. Lots of such services sell bots and when someone will check your subscribers he'll see that they're fake and it will be the loss of money, nothing else. No one would believe an account with lots of fake subscribers.
Buying bots is not the best idea for promotion. Now Instagram is very closely following this. You could try promotion services for IG. There are those with which you can accumulate real followers.