DO NOT speak to your RAOWO mate, he will only know the answer if he has been a property member himself !!! Go through your Regimental/Corps HQ mess, or dig out a receipt for the last book to see where it was bought from !!
The book will need to be purchased from mess funds or PRI if its for the Unit.
Why not drag the Regiment into the 21st century and have your property book on CD. After all, MOD and other high formations have converted paper books and manuals to CD, so why not your property book. It is also easier to maintain and you can produce a pretty label showing the Regimental crest.
If you ask around, you may find that it has already been done by someone, somewhere, who might give you the Db or spreadsheet already formatted to save you a bit of work.
Sounds like an outstanding project for an aspiring JO out to impress...
If you want to have an electronic record of your property book with nice pictures so that you can identify items, ring the Churchill Officers' Mess in JHQ Rhiendhalen. They have a programme that prints out the property checks and Bar Codes everything - made my checks very easy :0)