I recently bought a battery which is pretending to have 1600mAh capacity. It is sold by Amazon itself, not market place:
http://www.amazon.de/dp/B002HCTZCI
It was not at all charging or recognized by N900 and I had to bend the connectors at the battery a bit (like xman suggested in its post from 2014). Since then it is working stable, with OK energy lasting. I think, it doesnt notably last longer than a 1300mAh one ...

So this battery has my recommendations to other N900 users ...
Thanks to "newer" Nokia Lumia 520/521 mobile phones it is at least possible to buy real BL-5J batteries nowadays, without holographic thingie though.

Side note: If you order from Germany, you can save shipping cost by additionally shopping a cheap book (the more boring the better) and cancel this item when the battery is delivered.
Use this link for an ordered list of cheap (and boring) books:
http://www.amazon.de/s/rh=n:52044011...price-asc-rank
The price is secondary, more interesting is that amazon doesnt have it in stock ...

Cheers, Ulle