Well, we're having some problems with terminology, here. When you say "vintage", do you really mean "used"? And when you say "longarm" do you really mean "midarm"?
Longarms generally refer to machines that are 16" or longer. Midarms are 9-14", and DSMs are what you've been using. Brother came out last year with a new model that has an 11" harp, but since it's a newer model, I doubt you'll have an easy time finding a used one at a good price. I believe there is another manufacturer that makes an 11" machine but I don't know who it is. Bailey makes machines that are 13", 16", and 17".
If you're searching online, maybe changing your search terms would help you find what you're looking for.