Take a look at your flywheel. How is it drilled? 3 sets of closely spaced holes, or 6 equidistantly spaced holes? The ones that Jim posted from VanPelt are Long style with the 3 sets of closely spaced holes I believe you will need a Borg and Beck with the 6 equidistantly spaced holes. I know of nowhere to find a new one but Ft. Wayne Clutch (800-258-8243) can supply you with a rebuilt.
wilcap has the borg beck if you need that type he has them special made,i have one in my olds powered merc works perfect
A much more common, cheaper and better diaphragm clutch can also be used in place of the '49-'53 B&B that Mercurys used. Most clutch re-builders can match the finger height and provide the correct replacement.