I took the factory o-ring to Seal Source (an O-Ring and Seal specialty shop) in Portland, OR. They couldn't match the factory o-ring exactly, but they gave me their closest match which had a slightly smaller inside diameter and was just slightly thicker in the cross section.
They said the o-ring size was 4 x 8 mm. ( I think it's a 4mm ID, 8mm OD, and 2mm Cross Section )
I replaced all four o-rings. One at each shock, and the two on the air pump. The two in the shocks were pretty hard to get the white tube back in the fitting, I assume due to the tighter inside diameter/cross section. When I pushed the fitting into the shock, I could feel the o-ring springing the fitting back out a bit. I did use some of the TFE paste on the white tube that inserts into the fitting / o-ring. I tried both with and without TFE, didn't make the fit any easier without, so I decided to stick with a little TFE paste for extra seal. I eventually was able to get the tubes to slide in on the shocks. The Air Pump fittings seemed to fit fairly easy, even with the new o-rings.
The air seems to be holding, haven't checked with a trailer on yet.