Just my 'sperience, Lucygirl ---- FWIW:
I tried the source (for Borg Dually Valve Stems) mentioned in post #5, but with my Alcoa wheels they wanted to know what angle I wanted. I didn't have a clue so, at their suggestion, I called the folks here ... and they knew exactly what I needed.
I took the hardware I rec'd to Les Schwab Tires and they installed the Borg stems and stabilizers for about $85 IIRC. They said it was a simple job cuz they don't have to dismount the tire ......... only break the bead.
I can now check the tire pressure/add air to either of the duals without even getting down on one knee. LUV it. Kind of pricey, I guess, but it is "once and done" for sure.
Please know that the Borg stem on the inner dual is so long that it is nigh impossible to spin balance that tire. I took a pair of "Centramatics" to Schwab at the time of the stem installation and had them put them on ..... so no dealer-balancing needed. Possibly overkill, as many owners with our 16" wheels do not balance the duals at all ..... just the steers.
Good luck with your project. Stay safe.