Duggal in New York might be able to do it, and I'm fairly sure that LTI in New York can handle up to 20x24" dip and dunk. I've personally seen the the sheet film holders in 20x24" for a Refrema at Sixty-Eight Degrees lab, which is a sister lab of LTI, and now they are in the same facility--68 deg for B&W and LTI for color, so they can probably do it. They both have websites, so you can google their contact information.