Well the NME manual
doesn't list the orings that are on the barb.
I took mine apart to look at it and there are two orings. One between the barb swivel body and the screw head that holds it in place and another between the swivel body and the LPR body on the marker.
The o-ring between the barb swivel body and the LPR body is 1x4.5 mm. The other oring underneath the screw head measured 1x4 mm when I took it off, but it may have been stretched out from removing it. I took a 1x3 mm oring and put it in it's place and it seems to fit. I do not have a 1x4 mm oring to verify.
I needed 1x4.5mm and 1x3mm orings for the solenoid manifold on my Bob Long Vice and I could not locate a source to purchase them anywhere. I had to contact Bob Long and they sent me some out. You may have a difficult time finding those sizes. Hopefully not.
If you do, your best bet is to either get the Palmer Odessy LPR as suggested. I have one on one of my NMEs and it's awesome!
The other option would be to replace the barb with another 10/32 barb as it would come with the o-rings you need. You can get one cheaply here from airsoldier. I believe Palmers also has several of them.