Scoring for Brass Band
You could try a few secondhand book sites - www.abebooks.co.uk or www.bookfinder.com or even Amazon, because they do old stuff too.|
There is a bookshop in Cecil Court, Covent Garden, called Travis and Emery (I think) who do just secondhand music books. There used to be one called Decorum Books in London somewhere too.
You should be able to get it via an interlibrary loan through your local lending library, though, even if it is out of print.