Double Tap is know to be weaker than claimed as are a few others.
Underwood tests show it to be consistent. I like HPR ammo for 10mm as well.
http://hprammo.com/product/10mm-auto/
Another place is Georgia Arms.
You should really consider reloading for 10mm. The full house loads or just buying factory ammo in general is pretty pricey.
A basic reloading setup would only cost you about the cost of 5 boxes of ammo.