You shouldn't have any trouble finding the fittings you need at any home improvement store like Lowes or HD. You will probably need to go between the plumbing department's selection of brass fittings and the tool department's selection of compressor fittings to get the threads to match up.
I find Lowes has an excellent selection of brass fittings in the easy open
packages. Mine took about 4 different fittings and I sealed the joints with Teflon tape. Works like a charm for winterizing.