You can buy one (printed) from CJE Micros - something around £30 if they have any left.
Alternatively you can buy the DDE from ROOL http://www.riscosopen.org/content/sales/dde
for £48. This has the 1992 edition of the BBC BASIC Manual in PDF.
There is a CD 'RISC OS Programmer's Reference Manuals in PDF and HTML formats' originally published by RISC OS Limited, which you may find on ebay - this includes the BBC BASIC Manual.
ROL is now defunct but the web site continues to be maintained by Aaron and he intends to make the BBC BASIC Manual available, free, in HTML format in due course.