The best flux brushes are free! by using a feather and cutting it as shown - makes the cheapest and greatest flux brushes ever!!!!!!!!! Im sure all the old jewellers will know this but Im sure there will be some new jewellers who will be pleased to have learnt this tip. http://www.ganoksin.com/ftp/catalogo-8.jpg
Great tip! Do you have any suggestions on the best way to clean the
feather? I would assume that removing dirt, mites, germs etc. is a
top priority before using. Or I guess you could just buy dyed ones
from an art supplier but I say where’s the fun in that? 
I know many calligraphers still cut and use feathers for detailed or
traditional illustration that mat be another resource for info on
use, and prep techniques.
Cheers,
Taylor in Toronto