Flowserve makes parallel slide gates and has custom made a large special valve that functioned similarly on a project I worked on... (I am not endorcing this company, only that I know that they do make this size of valve)
I am sure that here are other mfg. that can also provide this valve -- try looking in Thomas Register for vendors...
You might want to consider using a 24" valve. They would be more commonly available and if you check the pressure drop across a 24" versus a 26", it's not going to be much more.