You're probably going to have to phone around a lot of a secondhand book shops. That book is out of print, and I can't find a copy of it on any of the usual websites. It's by Hilda van Stockum (1908-2006), and was published by Constable Young Books in 1963. (It's not to be confused with the later 'Jeremy Bear' series by Ursula Hourihane).
Regrettably (other than phoning lots of secondhand book shops) all I can suggest is regularly checking these two websites (and possibly Amazon as well) to see if a copy turns up:
http://www.abebooks.co.uk
http://biblion.co.uk
Chris