Sleeving the dollar cards in Ca$h 'n Guns where the exact sleeve size doesn't exist
Putting the comments in an answer so we can "close" the case.



Honestly, I would just buy a second copy of the game. You're talking about a potential cost of $15-20 (for double sleeves) to protect the components that are handled the least (besides the diamond, of course). Furthermore, damaged cards would have almost no impact on the game, unless you're playing with card counters... but then just shoot the card counter!

The cards are certainly essential - they just aren't handled that much. Each card is shuffled at the beginning, dealt out and chosen during one round, and kept in a player's "stash" until the end of the game.