Why not try a local engineering or fabricating company? Something like this is a very simple job, and you'll probably get it at a much better price than buying them from a "proper" marine company.
I don't know if either of these is actually any good for what you want, but a couple of minutes on Google found these companies local to you:
Croft Engineering
www.croft-eng.co.uk
Day's Fabrication 01420 588070
Might be a start . . .
John