If you're not particularly picky about what its looking like you can get replacement UMDs such as
these.
Sometimes you can get lucky if you check out your local game store and they'll sell them to you so you wouldn't have to order them. We carried them where I used to work simply because sometimes we needed replacements! If you asked nice enough we'd replace your plastic container for you for free since each pack usually has like 10 and I doubt you need that many.
Though just a caution about the replacement ones - they're a bit 'bulkier' than the ones typically found on PSP games, so fit much more snug than what you'd be used to in your PSP.
Again, like what someone mentioned before - replacing it with a normal one would be difficult to get off since they kind of hinge on each side and may be difficult to pry apart by just your hands without damaging them.
