A Thunderpower 3s 3500HV is under 200g.Nice to here that you are going for a battery mod, which one did you go for?
When it comes to which external battery to use, that is the million dollar question...[emoji23] im currently experimenting to find out, but my longest flight so far was made with a 2200 mah at 200 gr.
Where to get them is to Google for a vendor, i bought mine in Sweden, where i live...what you are looking for is a 3S LiPo or LiHv with as much mah's as possible but at a weight at or below 200 gr.