I have for sale one box of Willson R553 replacement filters.  Actually, the listing is for one box but I have 9 available.   Each box has 10 filters.  Part R553, for use with Agri-Tox respirators.  you may get a different design on the box you buy, but they're all R553 filters.  You can't really find these filters anywhere online from what I see when I search, so don't miss this opportunity to grab a bunch of them now!