... Obviously, I can't just take it down to my local Mercedes dealership, ...
Why not? Why would you not be able to take it to your local MB dealership? Just give the local ones a call and ask.
They're the best to service MB vehicles and the Metris is only a light commercial vehicle, the size of a passenger van, so they don't have to have the large 6x commercial bays with multiple extra heavy duty lifts, that a full on commercial authorized dealership needs; to receive the biggest commercial Sprinters with a full load of service parts and equipment.
In our rural state, our closest MB dealer is probably one of the smallest ones in the country, and they service Metris & Sprinter, no problem.
Perhaps some dealers in other states can specialize in passenger only, but as the Metris exists in passenger version and is just the size of a big minivan, I really don't see why any MB dealer couldn't service it.
My experience with indy shops has been mixed. The largest most reputable in my rural state area is more expensive than the dealer - perhaps because they specialize on the luxury, super sports and antique side of things on wheels - and they required an extra intake visit, the two times I contacted them about service ...
One tiny 4 person shop has a mechanic who I like, and has worked on our MB off warranty ... one can save because it is not dealership shop rate; but parts are through their vendor only and tend to have a serious markup ... up to double and more ... relative to buying genuine mb parts online from parts warehouses.
Good luck!