Best solution for 1440x900...and other 'widescreen' resolutions is to take the plunge and learn how to crop/alter whatever wall you wish to use, whatever its res.
I recently had my monitor die...it was suited to 1024x768...and its replacement has a default [native] res of 1680x1050 ....equally widescreen in format as 1440x900.
I resized my existing 1024s to 1680 ...and cropped to suit.
Generally they will work just fine, though some may lose bits of the image you'd rather they hadn't.
The problem is, as others have mentioned...they provide resolutions as suited to their own format/s as some may simply not be able to reset their screens to these other resolutions to verify results...