T.R. Sullivan says the Rangers are out of the running for Tazawa, and it appears he'll be going to Boston. Sullivan also says:
The Red Sox have made it clear that they aren't interested in trading Clay Buchholz under any circumstances
So, take that for what it is worth.
The Boston Herald says that it will be a week or two before action on the Red Sox catching front gets serious, with the team and Jason Varitek apparently still having discussions. The Herald says that, along with the Ranger catchers, the Angels have expressed a willingness to part with Mike Napoli or Jeff Mathis, and there's a rundown of other catchers that are supposedly available to Boston in the article.