I lost my o-ring also. I just made a spacer out of some plastic. Most any thing will work for a spacer it`s not much of a gap that needs filled up.
Yup, I wouldn't sweat trying to track down that exact o-ring. CC probably had a bunch of those extras or sourced a cheap supply.
Bunch of folks over on EDCF just cut a rubberband down to size and shoved it in there.
If you were really wanting to be careful, you could put a dab of glue between the end of the Fisher refill and the spacer (rubberband or otherwise) so that you could always be guaranteed to be able to get it back out.
Main thing is to use something squishy so that the tip has a little give.
Also, if you stack two pieces of rubber band, then the tip will protrude a little more. Some people like that because it gives them a clearer view of the page. Like if they're drawing, for instance.