The November 2013 LVA newsletter has an article about Win Cards. I did a run and found a couple of other opportunities:
1. The Luxor has Win Cards: 30 in chips for $20 available at the cashier (even WinCards.com doesn't mention The Luxor).
2. Circus Circus offers 45 in chips for $30. I've never seen that one before. Start at the players' club near the front entrance.