Pre-war BSA M24 Spare Parts
As we are dealing with bikes that are 80+ years old, New old Stock spares are in very short supply, and anything available tends to be expensive. M24 specific items in good condition, such as Cylinder heads & Barrels trade for £1000+, and some cycle parts such as rear stands and fork yokes can sell for up to £200+.
Many cycle parts were common to other 1937-40 pre-war M-series models, including the war time WDM20, and one or two specialist dealers still have stocks of new WD parts.
Copies of the original parts books for 1938 & 39 BSA models are an invaluable asset when looking for spare parts. Reprints are still available from the National Motorcycle Museum bookshop as well as copies of other useful BSA literature such as Instruction booklets for individual models. These will provide illustrations of all parts with original part numbers. 

Factory Drawing of 1937 Pre-production M24 engine
www.russellmotors.co.uk in Battersea, SW London provide a useful downloadable catalogue and price list for the WD M20 and keep a good stock of suitable, and often new old stock parts.

www.draganfly.co.uk in Suffolk, UK also keep some pre-war BSA parts. Again you can download a very comprehensive catalogue full of useful drawings and part numbers for early Gold Stars, and a price list.

http://www.cornucopia-enterprises.de/ based in Germany have WDM20 and some 1930s spares, again with a comprehensive online catalogue

I keep a stock of parts and occasionally I will have new reproduction parts made using original patterns and may be able to help. See For Sale & Wanted page

 
M24 Petrol tanks made in India - BEWARE - These may look good from the photos, but one recently sold to Australia had the front brackets in the wrong position,
mounting holes for kneegrips & badges wrongly spaced, and it did not fit properly. They may be cheap, but you get what you pay for.