There's a guy on nintendoage that makes very nice ones. I ordered some from his website, the name of which is unfortunately not in my brain at the moment. I've also ordered some through eBay (where he also sells them). The prices are very reasonable. EDIT: Found it http://www.videogameboxprotectors.com/store/4569532134I would highly suggest the "NES Box" size for NES games in the box. They fit perfectly.
I bought my protectors from them. They are really nice. I have all of my boxed Castlevania games in them and traded my extras away. I thought I had an extra one, but I'm all out.