I looked at this for my old steel bike (loved it, but the frame was a touch too small for me). If the brakes are nut fitting, (i.e. the fixings aren't recessed into the fork and brake bridge) they're hard to find, but not impossible. Tektro manufacture some reasonably priced ones, and I found a place called Byers Cycles (they have a website, but do mail order by post rather than true online shopping).
The other thing to consider is the "drop[1]" of the brakes, measured from the centre of the nut down the the middle of the braking surface of the rim. Older bikes tend to have longer drops than newer bikes (although this isn't always the case).
[1] My mistake, this is actually the "reach" of the caliper, see
here.
Byer Cycles Brakes Page;
http://www.byercycles.co.uk/02R0908.html