12x18? That's out of my league.
When I was in business I had an HP laserjet III. It cost about $1200 and was just b&w, but what a workhorse. It slaved day after day for years. We've had our best luck with HP printers but I couldn't see any on the HP website that said they'd do 12x18. My printer is an hp officejet 6610 "all-in-one" I bought a few months ago. I love it. It can handle 11x17 and has 40 buttons on the front. I could get into serious trouble messing with 'em. <grin> It looks like this:
(Really, jp, I can't help you but I just posted so I could say hi!
Good luck in your search.)