I'd try 'phoning round your local model shops.
There is one fairly close to me (Derby) that often has loco bodies in at reasonable prices (LMC Derby), but I wouldn't hold your hopes up for a Silver Link body - From what I can ascertain, they are somewhat akin to finding rocking-horse manure.
That said, there are various Hornby spares retailers on the Internet; a quick Google search brought up numerous listings.
You could also try the various railway modelling forums, New Railway Modellers, RMWeb, etc.
It may be easier, cheaper and quicker to purchase a 'standard' A4 tender body and paint it to match the loco!
EDIT:
You could try here:
EAST KENT MODELS
89 HIGH STREET, WHITSTABLE
CT5 1AY
Tel: 01227 770777
Don