Read This First
If you are ordering a bidet, you can add an extra O-Ring for $0.25. If you only need an O-Ring because you want your bidet to fit a different bottle, check the ship-one-O-Ring option and it will run you $1.
 
Everything below is optional reading.
 
Why won't I give out the size?
People write in asking for the O-Ring dimensions so they can grab one at the hardware store. Store-bought O-Rings crack after about a year. Not a guess. That is where I started, and I left mine outside year-round to push the experiment.
 
You can also source the exact same size and the bidet still will not work. Not always true for the NOBO or CNOC bottles, but for the standard bottle O-Ring it is very true. The O-Ring I use comes from one factory in the Philippines and something about how it is made produces just the right amount of friction. Nobody at the company I order from can explain it. Once they filled my order from a different factory, same exact size, and they did not work. I keep a three-year supply in my basement now out of fear some President might come in and mess with international trade :)
 
They are also food-grade. Non-toxic, chemically resistant, safe for direct food contact.
Strong too, with one exception. My mom mentioned the CNOC O-Rings were breaking on her occasionally. I tried to snap one myself and could not do it. It turned out it was her nails. One small nick and the O-Ring will snap when you stretch it on.
 
Why does one O-Ring cost a dollar?
$0.80 stamp
$0.10-$0.28 per O-Ring
$0.08 Envelope
$40 buying bottles to test compatibility
Unknown, driving to hardware stores
$5 paying my local hardware guy for a single O-Ring because I appreciate him being there
Unknown, time spent tracking down matching size and correct type
Unknown, extra fiber on day one so I could go on day two and test the seal
 
One dollar is a pretty good deal.