Good luck in finding what you need. I sent a late-night fax to the Rostra Tool Company. The next morning I was awakened by a phone call from Rostra. He said they are relatively expensive because they require a good deal of machining. He suggested they might be available from the McMaster-Carr Supply Co. of New Jersey.
Several sizes are available.
McMaster-Carr sells in any quantity and provides several means for ordering. There is a loose-knit community of antique tool fanciers providing a variety of resources on the Web. Probably one or more of the tool brokers might be able to watch for a used pair of such parallel pliers. The OldTools mailing list seems very useful. Unfortunately, to post a question requires joining the list and therefore receiving around 100 messages per day.