All good things must come to an end. We have stopped carrying the T-Rex battery simply because it does not offer anything special (other than a different light pattern) and there are more dependable batteries out there.
If you are looking for exactly the same size battery, please purchase a Joye eGo battery. It is also 650 mAh, looks essentially the same and has the same threading on it, so it will work with whatever atomizer and charger you were using with the T-Rex battery. The only difference is the LED light. The button lights up on an eGo battery instead of having the circle of light that the T-Rex had. That circle of light pattern is what caused all of the problems with the T-Rex battery and made the posts so loose that they would stop working far too often.
If you want 50% more battery life, consider purchasing an Evolution-X battery. The 900 mAh Evolution-X batteries are about ¾ inch longer than the T-Rex battery, which is why they last longer between charges, and they also fit the same atomizer and charger as the T-Rex.
Finally, consider purchasing the best battery of them all - a SMOK Winder battery. The 1,300 mAh SMOK Winder batteries last the longest of all the batteries we carry here and also fit the same atomizer heads and the charger we sell with the T-Rex on this website. They DO NOT fit the 5 battery e cigarette charger sold here, though. Best of all, the SMOK Winder batteries have adjustable voltage, which allows you to control the amount of vapor your e cigarette produces. Once you have used a variable voltage battery, you will NEVER go back to a non-adjustable battery again.
We are sorry we had to let the T-Rex go, but progress means change sometimes. We're sure you will get more useful life out of any of the batteries mentioned above!