It won't help you to/from LAS, but any airline flying an Embraer 170 will have the widest and most comfortable coach seats around - 18.25 inches wide, nice padding and good elbow space.

The Midwest example is good (although techically FC) - but if you book them, make sure you don't choose a "saver" flight. Those aircraft have regular 2x3 seating with 17" wide seats.