I don't know of any 640x480 skins.. but you can most definitely use any skin at 640x480.. just edit the general.ini file in the skin's folder and change showheight and showwidth to 480 and 640 respectively. It won't look as good but then I don't think anything looks good at 640x480..
Regarding making a skin, there's no skin maker tool.. But it's a matter of making the images and writing down the definitions (.skin files) for each screen. There's a summary of definitions for skins in the "skin commands.txt" file. Other than that, if you simply follow the examples of other skins you should be ok..