For all of your loose cartidges you can buy these cases for them and you download and print these covers off and put them in the sleeve. Its like dvd cases but they hold n64 cartridges and your printing the cover for them.
Cases
We are using the generic game cases from MediaShelving.com. They can be bought cheaply, but only in bulk (100 at a time). So, unless you have a ton of caseless N64/SNES/Genesis games, you will probably want to have one person buy them and then split the cost.
They can be found here. Make sure to get the ones with Full Sleeve (to put the cover in, of course).