G2 Battery Help

I have a Tmobile G2 phone.
The battery is starting to be really bad.
Like it'll be charged fully, but it'll run out of batteries within half an hour of use or so.
It happened to me for my G1 phone, and I have no idea why.
I'm thinking of buying a new battery, but is it possible to buy a different battery from the original battery?
The original battery that came with my G2 phone is a removable 4.81 Wh (1300 mAh) Li-ion (At least that's what it says on http://www.anandtech.com/show/3967/the-tmobile-g2-preview/4).
If not, where can I get a battery like that one?
If I can, what battery should I get, and where?

 

I would go to your phone carrier retail store and ask them what is wrong with the phone. Next time you get your new battery, completely use it up before you charge it.

 

Search G2 Phone Battery on Amazon. See if any of those possibly might be able to be used