According to the the Bone Man at NextShooter.com, here is where and when you can find the tubs:
One Tub at Wild, Wild West (probably open only evenings, weekdays, and on weekends).
One Tub at Ellis Island (probably open only evenings, weekdays, and on weekends).
One Tub at Circus Circus in West Casino section, hardly ever open unless on busy holiday.
2010 update: I hear the Ellis Island replaced the tub with a full craps table.