Not sure if this outfit is still in business, or if you want a repro, but you could try:http://www.users.globalnet.co.uk/~thinr ... esses.html
Note, original spares do come up on ebay from time to time usually described as for the blue cloth (home service) helmet, and the brass fittings are the same as for the FS helmet. There were three basic spike bases for the FS helmet: a round dome with 3 vents; an acanthus base; a cruciform base. Some FS helmets had white metal (silver) fittings, and some brass. Not sure if this is a solid rule, but roughly it seems that the fittings often match the colour of the helmet plate.