Wow, lots of talk about this little matter.  I just went through it myself on 08 Fat Boy.  Being kind of remote north of Saigon I only found oversize o-rings.  Spent a sunday afternoon sanding them down to make one that finally fit. Thought it was holding, but overnight it started to leak again.  Into the city a bike shop had some that worked.  Not exact but holding.  
Singapore shop only sells complete check valve with the teflon fuel line $130.  Same story from HD in Texas.  So got to do what I have to do.  Will change out on arrival.
Anything with moving parts is going to wear, but this is a matter HD should address.  Either improve it, or make these little rings available.