As you already suspect you are going to have a very hard time finding much in that size range. Here are a few websites that offer that size range but bit styles are limited. If you are able to afford import prices you will have more options in the UK, but still it's very limited.
shipshewanaharness.com (Mylers through a 6") http://www.drivingessentials.com/sprenger_bits.htm (has a conrad 3 slot liverpool -- not dressage legal, but for all purpose riding simply put the reins on the bottom hole. Wow that baby is expensive.)
Curiosity. What type of horse? What's your reasoning for wanting this particular style of bit?