well i found this case on ebay and it seem pretty cheap and plus it rubber so putting it on won't scratch your N900, and they case seem pretty good in the pix and seem to fit the n900 well, i just put it on my nokia n900 and its perfect, very nice fit and very easy to put in, now the only problem you might run into is that it cover the whole back of the nokia n900 so when you want to open your battery to get the sim card out etc, you would actually have to remove the bottom case but it come off pretty easy so no problem there ^^
http://cgi.ebay.com/Black-Rubberized-Hard-Case-Cover-Skin-For-NOKIA-N900_W0QQitemZ170402259566QQcmdZViewItemQQptZPDA_Accessories?hash=item27acc4226eif u can find any better case, please go ahead and post ^^