I've got a mate that does plastic -
http://www.k2a.co.uk/ Give Ken a call as he rides a 'blade and knows about the racing.
He'll sort you out if you need any sort of plastic.
As for metal, you can pick up plenty of great bits of metal in B&Q - little brackets and stuff are well easy to come by in there if you hunt around.
They may not be cheaper than buying from a merchant, but you bits in various sizes in decent finishes.