Re: 34mm to 30mm ring reducers?
I'm with Rob on this one, some companies make them, not sure if anyone makes a "universal" version.
For me I'd rather have as few of parts in the system as possible, so if it's top end stuff you can probably recoup most of your cost by selling the old mounts/rings and buying new correct size. If it's low cost, the steel TRS rings are pretty reasonable and the Warne rings while a little different are really solid as well and they probably cost less than some of the adapters out there.