Hi, Dean. I've used ACP Rail for my JR passes since 2011 (and also for BritRail passes! they are the official vendor for those) and never had an issue. You'll get your exchange orders in like a week or so IIRC via Fedex. They're based in Canada.
There are several different online vendors, though, and all should be about in the same price range. I'm not sure there is a single "official" vendor for these, either. JTB might be but not positive.