Hi Kevin -- thank you for posting to History Hub!
The easiest way to search for congressional hearing testimony is via ProQuest Congressional, a subscription database. I'd recommend checking with local academic or law libraries for access to the congressional hearing module within ProQuest.
Congressional hearing transcripts can also frequently be found freely available online through a variety of sources, such as GovInfo, the Internet Archive, Hathi Trust, Google Books and the Law Library of Congress.
I was able to locate Eric Johnston's testimony on March 27, 1947 online through Hathi Trust. It was printed in a hearing titled: Investigation of Un-American Propaganda Activities in the United States, which took place on March 24-28, 1947. His testimony starts on page 1082 of the whole scanned volume (and it's page 288 within the hearing). The hearing is available via Hathi Trust within a series of scanned HUAC hearings from 1947.