Glad to hear you're going to start the conversion to the Sharon couplers. Sergent Engineering does indeed sell a 144 count pack of Sharons. It's item # ENC87KB, and can be found on the narrow shank link on his website. If you haven't already, I suggest picking up a copy of the HOn3 Annual from 2014 to read the article John Niemeyer, Mike Conder, and I wrote about the Sharon couplers. Here's the link if you don't own it:
As far as coupling compatibility with other Sergent couplers, I don't have any experience with that. Hopefully someone else here on the forum can provide feedback.