I located the bag of o-rings. They were actually sent to me for replacements on drip tips. I think they are too big for the istick (1.5 x 6MM).
Aldenf, do you recall the size that you used?
I have #60 Danco o-rings (1/4" O.D x 1/8" I.D. x 1/16"). I took a bit of time and sanded the o-ring a bit flat on one side so, once installed, most attys sit flush with the top cap.
Those 1.5 x 6mm o-rings may work great! Not sure. The [HASHTAG]#60s[/HASHTAG] are a bit stiff and a challenge to stretch over the top of the 510 pin.
Good luck!