There has not been an "official" release for either system yet. So people have largely taken matters into their own hands. Some people resize the UGC covers to match the CGC dimensions, and others make all-new covers. For example, I pre-ordered 100 SNES cases in bulk for my collection and ended up making my own covers for every single one because I didn't want to wait for someone else to do it.
Wiggy has posted a thread containing his custom white N64 covers (I think he did the entire library), and I posted up covers for the SNES games in my own collection (including a template file for GIMP so people can make their own to share). There's also the very cool "Fusion" covers popularized by TyrannicalFascist. Take a look around and you may find something you can use.