Accordion-O-Rama This is a New Jersey retailer in the New York City metro area. They are direct importers and do repair work. They used to advertise in the back of Popular Mechanics and this is how I got onto them. I purchased a Hohner instrument and case from them and everything was fine.
Accordions and Keyboards John Gaunt has his store in Clearwater, Florida. Not as big an operation as some, but he has a good reputation for repair and tuning.
The Box Office Here's a shop specializing in Irish-style accordions and is a direct importer. Located in Port Jervis, New York. The site has a page of Irish-style keyboard layouts.
Button Box/Northeast Squeeze-In A few miles north of Amherst, Massachusetts, the Buttonbox stocks a lot of used instruments, does special orders out of Europe, does repairs, and manufactures concertinas. It's also the host of the Northeast Squeeze-In.
Castiglione Accordions Here's a store with a big inventory on the north side of Detroit, Michigan. John Castiglione has accordions made with his brand name on them and also handles many standard brands. He has a repair shop.
Hobgoblin Music A vendor of various types of instruments.
House of Musical Traditions Various types of instruments with a strong emphasis on accordions in a Maryland suburb of the District of Columbia.
Karlito's Way This Texas store is recommended to me by Otono Lujan of the Los Pochos group.
Lark in the Morning This outfit has a beautiful catalog of all sorts of unusual instruments with a small selection of accordions. Several store locations in California.
Reyes Accordions Here's a very nice Tex-Mex site, with a store in San Diego offering a variety of accordions and also bajo sextos--bass guitars used in Tejano music. There's some instructional material here for Tex-Mex style playing and also a good bit on the border culture.
Elderly Instruments This outfit in Lansing, Michigan, is mostly in the fretted strings business. But their fantastic color catalog contains a lot of other musical stuff, including a few accordions.