Just checking Google I found Mouseshoppe.com has 14"x18" posters.
Another site said that the World of Disney store has them but to get them framed at a place like Michaels (with coupon) rather than at Disney because they'd charge alot more.
Are you looking for originals?
I just found another, reprint posters - DL's Haunted Mansion & Submarine Voyage - on Craigslist for $400.00 for the pair. I don't know how to do a link but if you Google "Disney ride posters" it's on the 1st page.