Most of them are coming from Pakistan.

More than half of suicide bombers used by the Taleban for attacks in Afghanistan are not Afghans, the UN has quoted a Taleban commander as saying.
In a report on Afghan suicide attacks, it quotes the same man as saying more than 80% of the bombers are recruited, trained or sheltered in Pakistan.
[…]
Nonetheless, the UN quotes a senior Taleban commander as saying that more than half the attackers here are foreign, coming mainly from Pakistan and also Arab and Central Asian states. It says the commander’s views “have been verified”.
[…]
They tend to be poor and little educated, very often groomed in madrassas in Pakistan’s tribal areas.
The authors believe many attackers are opium addicts, and many others orphans.
The report says the authorities here are now preventing many such attacks and that even the ones that succeed tend not to kill many people. However, 80% of those they do kill are civilians, even though all the attacks appear to be aimed at Afghan or foreign security forces, or Afghan government targets.

I guess the Taliban aren’t committed enough to jihad to invest their life in it. Maybe the Pakistan government should broadcast that fact to the people of Pakistan to let them know that they are being used by the Taliban. That might cut down on the number of bombers.