I've never seen them offered for sale, but that doesn't mean anything. I've lost a couple of those O-rings and mashed another from over-tightening. The replacement that works for me is plumber's teflon tape. Cut a length sufficient to wrap around the threads, then cut in half lenghwise. It's easiest to do this with a steel straight-edge and an X-acto knife or scalpel against glass or a self-healing cutting mat. The teflon tape is too limp to cut easily with scissors. Wrap around threads, screw the Vivi Nova together, and you're good to go.
I envy your ingenuity but that is entirely too much to ask to do for a non-DIY guy. Everyone says how the VVN is "fill and forget", when in fact, it is not at all. Look at all the threads about them. I've had more technical trouble and goobledygook speak thrown at me about this VVN, when all I have to do is turn around, prime a carto, slide it into a tank, and done. Done, done, and done.
I am in the firm camp that anyone who thinks VVN are "fill and forget" simple are a little off the mark. It is most definitely NOT fill and forget simple.
EDIT: In order to do what you suggest, I need to buy:
1) Plumbers teflon tape (what is that and why and how, guess I'm going to Ace Hardware...)
2) Ok, we all have pocketknives, but what is a "steel straight edge", and now I have to get an xacto knife and extra blades from Ace Hardware, too?
3) a glass mat? A self-healing cutting mat? Where is the average joe gonna get one of those? even if he asks, he's not fully informed, so someone might sell him something else.
If I have to raid Ace Hardware to rebuild a freaking VVN, it's not worth the effort. Another toss.