I've purchased accordions from Liberty Bellows, driving from DC to Philadelphia. A few years back when I was considering an online purchase, they suggested I take the three hour drive, and I did. Whenever I'm in Philadelphia, now once a year or so, I make it a point to visit. Always welcoming.
Another member mentioned Petosa, and I've bought and sold several accordions that successfully traveled from the West to the East coasts of the US. Years ago I purchased an accordion from Music Magic in New Jersey, which no longer appears to have the inventory they once had. Another successful transaction.
The remaining accordion online sales outlets in the US can be counted on one hand. I have no doubt some smaller dealers-- one near Pittsburgh PA and another in White Plains have also remained in business in large part because of their reputations as legitimate retail establishments who serve their clients well. They have to be.
I believe this Forum does a great service to the relatively small world of accordionists-- in the US and across the globe.