You can get a Zippy 1S 240mAh 20C from HobbyKing HongKong.[$3]
Its about 25mm square and 6mm thick.
Weight is 4 to 5g, [simular to the 150mAh E-Flite]
You will have to buy with the battery, your own leads[mini JST] and solder them onto it, as it comes with tabs.
Be very careful of poalrity.
The leads come in a pack of 5.
Use one lead pluged into your charger permitily to make it easier to hook up battery to charger.
Again, be very carefull of polarity.[mark the lead plugs]
You will get about 10mins flying time from the 240mAh battery.
You should get 8 mins from a 150mAh battery.[bit expensive though]
The MSR burns about 20mA per min.
Just stick it on the bottom of the battery carrier with velcro.
Keep the centre of gravity[battery position] simular to the other batteries.
It looks a bit average, but works fine, cheap, and does not effect the Heli much.
If anything, like the 150mAh battery, it makes it nicer to fly with the bit of extra weight.
Good Heli flying is throwing it at the ground and missing