When I first installed my Stenner, I removed my puck feeder and used that tee with some added fittings. The puck feeder is threaded into that tee. Mine also had a set screw so watch out for that if you try to unscrew the body.
Pinch-a-Penny in Plano, TX has 10.5% for $37.12 for 4 x 2.5 gallon jugs before tax. As noted, you pay extra for the jugs first time through. Free jug on your 10th bought.
I sent an email to them via the HASA website for outlets near Plano. I got a reply back noting two wholesalers in East Plano. Neither appeared to have a retail storefront however.
I have the NS-48 which is the same, or at least in the same family as yours. Release the pressure, release clamp and give it time. You should then be able to twist the top half off.