I buy foam brushes, chip brushes, and acid brushes (for glue spreading) from an outfit called WiseAcre's Art. The quality of these cheap "brushes" is excellent. :icon_scra I buy dozens at a time. Their prices and service are outstanding, and they sell all kinds of stuff that woodworkers might be interested in. They are primarily a craft/hobbiest supplier. They sell on eBay, but also have their own website, woodworkingparts.com.
I have no relationship, personal or otherwise, with them. I'm just a satisfied customer.
Hope this helps.
Matt