You could contact Andrew Alden from bolex.co.uk
I see he has some viewfinders available. He's a really helpful guy and his prices are correct. Make sure you get one with 10mm attachment. I have reflex bolex bodies but I also have one, just to use it as a director's viewfinder.
Side notes:
1) Andrew also wrote a very practical reference book on Bolex camera's, called 'The Bolex Bible'
2) Can anybody teach me English? Is it two Bolex's, two Bolexes or two Bolexs?