Just check on what type of battery you are having in the TX. If I am not mistaken, the cheaper Futaba TXs use standard AA size cells. In that case it very convenient. Get yourselves the required number of cells from the local hardware store. Stick to a good brand that other have had experience with. I put 2300mAh NiMh cells form Chamelion in my 6EXHP and it never seems to run out of charge even during the week when I use it on the sim.

If, however, your TX uses a flatpack TX battery with a Futaba J connector, you will have to get this from a proper hobby store. In my 9CAP (it came with the same 600mAh battery), I replaced the stock battery with a Hydrimax 2200mAh and it lasts a good two weeks of daily flying on the sim before it needs recharging.

In both cases, get a high mAh battery so that it doesnt bottom out as quickly. You also need to note that the higher the performance of the battery, the shorter its usuable life. So keep an eye on the amount of flight time the battery gives over time.

Ameyam