Manufacturers do not want to make replacement from other sources that easy, regularly mixing two standard bearings into one assembly, although I doubt that Bristols went this far.
Take off one complete set of bearings (2 sets of bearings per hub; the inner bearing is usually larger than the outer; both will be tapered roller bearings) and find yourself a local machine tool service company. Take your bearings in and they will measure them while you wait (about 5 minutes). There are 3 grades of bearings used classed A, B and C. The tolerances in manufacture are, of course, extremely tight so they employ an automatic grading system – A being the best and that’s what you want. C are the poorest and probably go into washing machines and the like. I think the classification system may have changed possibly 1 – 3. If the numbers on the bearings are visible you should be able to quote these to any bearing company and avoid the need to go to a machine tool service company.
Valid at time of writing in October 2016:
There are a number of good bearing suppliers eg Seager Bearings, Kings Heath, Birmingham (0121 444 5391) who will supply at trade prices.