Charlie, you can buy entire
axle assemblies including the
brakes and springs.
I don't know how standard these are for width, but one place advertises a 3500 lb
assembly with
brakes for $470 cdn.
You can also buy the brake assemblies for $75 each if your hubs bolt onto the
axle.
There seems to be some degree of obsolescence associated with these
brakes.
Apparently the company that made mine is no longer in business and the magnets
to fit them are no longer available. With the hubs welded to the axles in my
case, I would have to replace the entire
axle assembly to replace the brakes.