AA works pretty well, after using iflystaff to book the eticket you need to list here:
Interline Fare Calculator
Choose the eticket option and find the flight you want. You can also get an idea of loads on this site. You'll need to input the eticket number from the company confirmation email.
Just a warning, the loads inside the US are extremely full every day. In the past four years they've shrunk domestic capacity to increase revenue, and there are a lot less seats. I've mostly used USAir and American, don't know how it is on the others. Virgin America will allow you to jump seat if there is a seat in the back, USAir sometimes as well.