I found some suitable brake bleeder cap replacements at Lowe's builder's supply a while ago.
In the specialty fasteners section (pull out drawers with all sorts of odds and ends), there are
plastic caps sold as "screw protectors" in a package of four that say "Fits #10 screw".
They were $0.92. I bought a pack and after trimming about 4mm off one, was able to press it
over the brake bleeder fitting for a nice fit.
eddie