I don't know how big the cards are for RftG, but if you go to your local comic card store/trading card store, you can probably find some themed sleeves. I remember about 4-5 years ago, they had some dragon themes, Amazon themes, angels, and some abstract themes. Not sure if they ever produced anything sci-fi like.
Just a warning about the theme sleeves, they seem to be printed on to the sleeve, so after much use, you will start seeing the images fade away. Also if you have oily fingers, the images will fade even quicker.