You need to buy 3s 11.1V battery packs with the ability to continuously deliver 36A under load. For the 1800mAH battery, 20C gives this power output. You can buy batteries with a bigger 'gas tank' i.e. more mAH and they can be had for less than the $62 that Horizon Hobby wants for their 1800mAH battery.

If you want longer flight times, then you need to buy a battery with more mAH. You'll also want to get a better charger than what came with the B400 so you can monitor how much you are draining your battery in flight. You don't want to discharge >80% of your battery's mA or you'll damage it. For an 1800mAH battery you wouldn't want to use more than 1,400mA.