Surely your local ford dealer has them or can order them in for you, brush wear is a common prob on the ranger, usually one brush wears right down to the bone while the other remains good due to the heat dissapation along the rotor shaft. I would give your local autoelectric dealer a ring or If you get desperate to find any in the UK, you can order them from NAPA Autoparts in the USA, its a small part so will not be too expensive for postage... part number F424 is the common kit I believe but check up with them first.

Try Sahib's Auto Electrics on the Hanger Lane Gyratory. Contact details are only a web search away and they're worth a bell.