Those cases should be pretty easy to find, although there's a good chance they'll come with another copy of the game
As you collect more games, and find that you have some expensive titles missing cases... a good alternative is the reproduction labels that Sparky made. They look exactly like the originals (and often times better due to the lack of fading and wrinkles) He charges like $40 something for a complete set for all the US hucards and CD games.
I feel bad because I just sacrificed a few cases for those said games today in order to replace missing cases on some more expensive titles