You may not recall but a couple of years ago some of the best known companies had to recall the original batteries as they were catching fire. One even caught fire sitting on a table in the middle of some exhibition in somewhere like Japan.
If you buy a decent make of battery then as far as I am concerned you wouldn't have a problem but I personally would not buy and use a cheap unknown make from the likes of Ebay.
You will be able to get an official battery from somewhere but its likely to be at least twice the price maybe more.
Hopefully someone who has bought replacement batteries will chip in with where they get theirs.