There is a french constructor who sell a 3000mah battery for N900 for 34,90 euros but they extended the thicknesss of the whole back :
http://www.maisondugsm.com/a/produit...okia_n900.html
Some image of their battery for N97 :
http://www.superetrader.co.uk/getima...pg&w=400&h=400
http://www.likwaiw.com/images/likwaiw/NKN97BL.jpg
http://image.pushauction.com/ViewPic...9-e1adb6bee83b
http://i289.photobucket.com/albums/l...endBlacktn.jpg