Chris is right about papier mache being very messy,and I don't think it would have the required strength if made as a shell over a mould. A better idea would be to do as Chris suggests and make a plasticine mould
and make the helmet from fibre glass. This is far easier to use,and is availiable from Halfords or similar car parts shops. It is sold as a kind of material or felt, which you cut to shape with scissors, brush with resin which hardens it. A few layers over the mould should do the trick.
The teddy bear helmet Chris suggests is really not very good. I saw one only Monday when I was in Hamleys. The company is Build-A-Bear. They have outlets in most towns. The police helmet is not solid.It is made of p.v.c material,so needs stuffing, and looks to me too small for most Policemen I have seen. It is part of a 3 piece outfit (helmet,tunic,trousers) price £12.00 Check out their website at www.buildabear.co.uk
as they do have some usefull things for anyone making puppets.
Hope this helps,
Kind regards, Billy Wand