Ok, I saw that many people keep going:
"omgz how 2 gt ammo"
And it is annoying me, so i decided to make this to tell you how.

WAY 1:
Right click on the weapon while it is in the Q menu, and click "Spawn With Toolgun"
Then aim at your feet and click, you will get ammo.

WAY 2:
Middle click the item when it is in the Q menu, this works the same as the toolgun.

WAY 3:
Download Spy's Customizable Weaponry addon and spawn the ammokit weapon, get the weapon you want ammo for, and stand close to it.

WAY 4:
Spawn the FA:S 2 Ammo from the entites menu.

WAY 5:
Do "givecurrentammo" in console

I really do hope has stopped the flood of comments that ask how to get ammo.
Thanks
-Pyro