I used to rebuild alternators - a long time ago.
If the "brush box" and Regulator are one.... replace the item itself - look on ebay... the brush box and regulator pack may only be 20 quid.
BUT..... and it is a BUT.....
If the windings and rotor have any damage, or are faulty, the brush box money would have been wasted.
To Summarise:-
If you are tight for cash, but can afford a fully reconditioned alternator - change the lot..
If you would rather "GAMBLE" that the £20-£30 brushes are the only cause of the fault, then change the box - keeping in mind that you may or may NOT have solved the problem.
Look Locally for a reputable Starter Motor and Alternator reconditioning company - they would probably ell the parts to you after offering to bench test the entire unit for you....
If it was me - with the Hippo I'm working on now, I would change the brush box (or what ever they want to call it these days) and take the risk that it is only this item that is worn. (because I'm slowly fully rebuilding the car from the ground upwards - and will only do local mileage)
BUT.......
The company I "USED" to work for would simply steam clean the alternator, and ONLY replace the faulty item... (which I argued against until I left of my own devices)
If I had the time while I was there; and not much work came in, I would completely strip it, Circuit test the windings polish the aluminium on the wire wheel; paint the casing and fit new brushes and bearings before bench testing. replacing the parts or rewinding as required.......
Ironically...... for some reason, the company isn't trading now!