I think - repeat think - that the major with the lowest CASM is either AMR or CAL. From memory both are about 9.5 cents.
The information is easily available, but it takes a bit of digging. It is rported in their SEC filings available under Investor Information (or similar) on each airline web site. It's a fairly long document.
Also, in the press release of their quarterly performance, the airlines will usually report their CASM
This site collects all the airline press releases, or you can find the press releases on the airline website.
Thye don't always report CASM in their monthly performance releases, though. They tend to save it for the quarterly.