I tried ITX when it 1st came out and in my opinion it doesn't come close to having the knock down ability that Nice Shot has. If you shoot all your birds at 30 yards or less ITX will probably suffice. As to availability, the company that developed it is Ecotungsten. Google them. They had all sizes avail a couple of days ago when I checked.
Nice shot is probably the best patterning non-tox shot. If you reload and load it yourself, you can use regular lead reloading recipes!!!! No special Wads or powders, excellent patterns and has a better transfer of energy or killing ability of anything other than lead.