Is that the built-in unscrew-and-connect-the-hose connector? Or is it a screw-the-adaptor-into-the-lower-unit type thing?
If the former, you should be able to find something at your local garden center. It's a female hose connector with a smallish barbed fitting. Alternatively, trailer brake flushing kits often have a similar part.
If the latter, I don't know.