Just re-read you post - Are you looking for a straight bar soft rubber mouth? There is a company that made a bit up for me once - Abbey bits I think they were called - can't find a website for them but they will make anything for you and they are not too expensive!
As someone above said, Abbey do make up pretty much any bit you could possibly think of. They arent the cheapest but when you cant find what you are looking for anywhere else they are worth it.
I know that Chobham Rider in Surrey use them. Dont know where you are based but they do have a website and do mail order. Their phone number is 01276 856738. Hope that helps.
the mare is very light in the mouth, goes well in happy mouth/nathe, but relaxes best of all in the rubber i think the flexibility she likes, i only use it in the school, been using a loose ring for quite a while and that seems to work well, i don't hack out in them but i find them good for schooling and i like the full cheeks for a little bit of poll pressure when you attach keepers she is a big horse and a big moving mare i thought i was bad a sitting trot on her until i found no one who can sit to her but that is another moan thanks for these ideas i will keep searching and contact abbey next week as she is 6 in mouth as well