I bought an airpack kit from northern truck or something like that, was ~85$ or so.
FWIW, I took my airpack apart, cleaned it all up really good with scotch brite, honed the cylinder (you need a cylinder hone, not a brake cylinder hone), put it all back together and the brakes work fine. It only takes about 15 minutes to pull the airpack off, and about an hour or so to tear apart, hone, clean and reassemble. I used RTV where I tore gaskets.
I threw the rebuild kit and stuff behind my seat, one day when i have an hour to spare I'll put new parts in it.