I'd ask a firm of marine engineers. They will know where to source one - looking on the internet for a machined piece of stainless might be a optimistic.
You dont say where you are, but if you are east coast I can recommend Brimbelow Engineering, Catfield, Norfolk
01692582707 I have has both shafts & props made by them.
No connections just a satisfied customer !
Any decent steel stockist who will normally be far cheaper than rip off marine engineers who get theirs from the stockists and add 100% IME. You will need to know what material you require as well as diamensions
Funny that, Brian: well recommended local engineering company quoted me £300 to machine up a prop shaft. Lancing Marine did the same for £80. Whose ripping who?