If mobs can not appear on islands how do I can get its drops (string, gunpowder, rotten flesh, redstone, bone glowstone...)? With mob spawners?

 

You don't

 

/visit mingo_x
He has all the public spawners you could need.

 

^^^

 

Thanks a lot.

 

I forgot.../visit Stel_ for guardian and blaze spawners, those two types are rare.

 

And I believe the only way to get rotten flesh is fishing