Hi Kaz,welcome to the forum
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You may need to be a little more specific on which parts you need, a picture sometimes helps.
Are you after the small cogs as shown (destroyed !) in the linked topic ?;
http://c5alive.co.uk/forum/index.php?topic=181.0If so, it would probably be easier just to buy a complete gearbox either new or used. Individual cogs for sale are quite rare.
I have a used gearbox available, email me if you are interested.
Which museum is your c5 located at ?