IsuzuParts.com is what I use. Others will undoubtedly chime in with Merlin at St. Charles, but I find that the price shipped is about the same. (From what I can see the advantage to Merlin is that he'll work hard to find stuff that's backordered / unavailable). But either one should have the regulators in stock.
There's also a link on the bottom right of the main page to a new contact for Isuzu parts. Jim something, I don't have experience with him, but you could shoot him an email.