Just as clarification, I bought mine for $20 through Wabfab, who just had them drop shipped from Trail Gear. Trail Gear sells them for $19 directly, so obviously he's probably letting Wabfab have them for something significantly cheaper than $20.
ARB sells a version for ~$11, that is the better way to go IMHO. Any Air Locker dealer should have those I'd guess. I keep forgetting to ask Ben when I talk to him.
But I occasionally flip through the pneumatic catalogs and see what I can find. ARB's 5mm is not a common size, although someone makes it. If a 3/16" fitting will work, those are dirt cheap (like $3).
"Silent enim leges inter arma." -- Cicero