Where to blow out line (picture)

[sniffmo]

5) For a shop vac, an SP1091-Z-7 works well (they're also called backwash adapters) If you're using an air compressor, they make expandable rubber plugs with a 'tank valve' (Like to fill a tire) that will do the job

Where can I buy this expandable plug with the tank valve? I have plugs, but have never seen them with a valve