Unless you have the means of time travel...
As the thread title states...
"Finding N900 batteries in 2016"
may be rather difficult.

Purchasing off the web is dicey ...
hit or miss...
I usually found it to be a miss...

However...
there are a number of decent ones for sale on ebay...
Just must be canny..
caveat emptor and all..
Some are in very good condition ...others are bold faced lies...
check the sellers feedback from customers to be sure..
That was the best way to trust the veracity of the seller and the n900 or n8x0 batteries they were selling...

All told I think of the 20 - 30 batteries I have bought from ebay for my n900's and n8x0's ...3 had poor battery life in them...
the rest were excellent.