I'm not much help with your questions, but I also think it's odd that this is the only Motobecane model with the Morini engine on it. Considering that Motobecanes have nearly the exact same engine on their many models over the years, I don't know why they went with the Morini on the Sebring.
I would imagine that you can get parts from any of the regular places. Check the "recommended moped shops" article. I'd start with Handy Bikes, as I know they had a large selection of NOS Motobecane parts when I needed them last.