https://www.ebay.co.uk/itm/Lots-1pc-HSS ... ctupt=true
change your search string, i.e. 10/28, 10-28, 10 28, etc (if that makes sense).
I'd still check the online stores, collectors will often keep half a tool and bits. Also, little bit of a tangent but the old Singer sewing machines used 10/28...