Hello, knotdonne. I can't help you by naming a specific repair shop in your area, but I CAN tell you what to ask of a repair shop once you find one, and this is important because many repair shops have never set eyes on an Interstate before.
You may have one visible leak in the commode area that you know of thus far, but chances are good that your 2007 actually has more going on than just that one breach, and if you take it in for repair, you'll want to get everything examined at one time.
Our 2007 Interstate, which we bought lightly used less than a year ago with only about 26,000 miles on it, proved to have the following plumbing issues (which were unfortunately not discovered pre-purchase despite the fact that we paid an independent inspector to evaluate the vehicle):
(1) Black tank vent line sheared off the tank, leading to slosh of black water out the top of the tank and onto the interior floor. Repair sequence described here:
THE INTERSTATE BLOG: FIXING THE BLACK / GRAY VENT LINE ON AN AIRSTREAM INTERSTATE
(2) Three lines sheared off the gray water tank. This was a bit trickier to notice because the gray tank is under the chassis and therefore leak evidence mostly ended up on the ground. Repair sequence in these posts (if these links don't work, search for The Interstate Blog on Blogspot and go to the Repairs tab):
THE INTERSTATE BLOG: INTERSTATE LEAKING GRAY WATER, PART 1: THE PROBLEM
THE INTERSTATE BLOG: INTERSTATE LEAKING GRAY WATER, PART 2: TANK REMOVAL
THE INTERSTATE BLOG: INTERSTATE LEAKING GRAY WATER, PART 3: TANK RE-INSTALL
^^ If you take that information to any reputable RV repair shop, it should help to get them oriented and give them a running start of what to check for with your model. We have the mid-bath 2007 but the general arrangement and construction materials appear to be the same on all of the 2007 Interstates that we've seen to date (mid-bath, rear-bath).
In finding an RV repair shop, if I were you I would avoid the urge to find an Airstream-affiliated shop and I would instead look for an independent operator. Local RV forums and review sites might be able to help you to identify one with a good reputation. I say this because, although my husband and I heard accounts of Airstream doing a lot of warranty fixes on plumbing on our vintage of Interstate, I would guess that there are no residual claims that could be made on vehicles as old as ours are now (the issue of whether these plumbing line breaks represent an initial product defect is another discussion entirely).
Hope this helps! Good luck with it.