I've used these before (the web site appears to have dissapeared though)
J Crispin & Sons
I think the address is :-
92-96 Curtain Road
London
EC2A 3AA
The website was :-
http://www.jcrispinandsons.co.uk/
Hope that helps.
P.S. Do you know what you're doing with veneer ? I didn't but found a couple of useful books in my local library. In the end I used glue film which works for me.