Finding pararell slide gate valve Diameter 26 inch for use in High Pressure steam .
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
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.

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