Can anyone please tell me how to find or make the hair texture alpha channel?

very simple, you can make it in 2 seconds!
1=Just open the normal hair texture bmp with photoshop or paint shop pro or another good GFX.
2=Go to HUE/SATURATION
3=Saturation must be set to the lowest possible, so the harcolour looks white!
4= Ready!

That's how I did it with buffon hair update and others!