Another relatively inexpensive source for boar bristles is a boar bristle shaving brush. Stiffer than a badger brush. You might have one of these shaving brushes around the house or attic. You can easily snip off what you need. Should be enough for a couple of hundred flies at least in one brush.
Just an idea. DO a search for boar bristle shaving brushes on google. Lots of places.
Stayed Dry in Plano