The following places are a good place to start looking for used books:

ABE Books   https://www.abebooks.com/

Thrift Books   https://www.thriftbooks.com/

Some new books can be obtained from here:

SNU (Spiritualist National Union)   https://www.snu.org.uk/

Harry Edwards Healing Trust   https://he-foundation.org/

Leslie Flint Trust   https://www.leslieflint.com/

Psychic News   https://www.psychicnews.org.uk/

Robert Goodwin   https://www.whitefeatherspirit.com/

Spiritual Truth Foundation   https://www.spiritualtruthfoundation.org/

SDU Publications   https://www.sdubooks.com/

Gyan Books India   https://www.gyanbooks.com/