All the parts are available from Dexter Axles <http://dexteraxle.com/brake_springs>. Haven't found anyone in the UK. If one brake isn't working, either the electrics have a bad contact, the magnet is worn out or the adjuster needs tightening up. All the brakes should be checked together and there are ways of balancing them so they come on together.
We have formed a growing UK Airstream group with contacts in Airstream Europe if you are interested. We had a little rally last weekend attended by 11 Airstreams from 1959
to 2008! Send me a personal note.