Try the Used Market

Like all books ever published, Murder In the Gun Room by H. Beam Piper should be available on the used market. If you're lucky, it hasn't become a collector's item (with the corresponding price inflation).

There are a number of useful sources for used books on the net, including:

Bookfinder	Search Bookfinder, a meta-searcher for used books
Advanced Book Exchange	Search for book
Amazon.com	Search for author