I did manage to find a HS Duo in 4.5" size at one point, but it had loose rings (much easier to find than a pony sized D). I think I contacted all the regular sources for pony tack without much luck. Maybe try emailing the contacts for Happy Mouth and HS directly? They could tell you if what you're looking for exists, then perhaps you could special order it in.
Myler does make pony-sized bits, so if your pony could tolerate a metal mouthpiece versus a plastic one, it would be your easiest source. Most tack shops have rubber mullen mouths in pony size as well (but again often only in loose ring). The upside is that they're cheap and you could make sure your pony likes it before splashing out on a $$$ custom order.
thanks guys, we have the mullen happy mouth loose ring right now and I know she likes the mouthpiece, but it's a bit big and we do show hunters and its a pretty ugly little bit lol. I am probably going to order something from Beris bits coming from Netherlands, but if I could find something cheaper and closer I'd be alot happier. I don't understand why Happy Mouth doesn't make their king dee mullen in smaller than 5 in!!
I have thought of Myler but she prefers a fatter bit, the rubber or happy mouth being best for her.
If you can get a good price on the King D (and the size of the rings doesn't dwarf her face), you could try getting your own custom bit made. I bought a cheap but pretty D, had the mouth piece cut off, and then a new leather covered mouth piece put on that was exactly the right size. The down side is that you need to replace the leather (or whatever covering you decide on) every so often, but it often works out to much cheaper than the super expensive ones from over seas.
I feel for you - I went through all the same things trying to find a pretty, soft little bit for my dainty-headed little horse too!
My farrier took the mouthpiece out and put a new piece of thick cable in, which I then had a saddler put a leather wrap around. All in all, I was out the $30 or so for the initial bit that I had taken apart, then maybe another $50 tops for the extra work to make the new bit. If you can find yourself a metal mullen mouth, a saddler could just pad and wrap it without even changing the mouthpiece.
Is this what you were looking for the Beris for? Keep in mind the Beris bits are very soft/flexible—they're not a hard rubber/plastic like a regular mullen mouth or even a Happy Mouth, so there's a chance it could be NOT enough bit for a pony that's used to a regular mullen mouth.