For mine I used one from Polarcell as they scored well in an extensive test for N900 batteries:
http://talk.maemo.org/showpost.php?p=872699&postcount=6

The first battery they sent me was however bad, it seemed the sim card wouldn't work at all with it.
But after I sent them a mail, they apologized and immediately sent a new battery in plastic bag with "tested" on it.
That one works just perfect.

Note: for many replacement batteries the connector cables are too long, but you can bend them so they fit within the N9.