These links are fairly specialized. Many accordion dealers do repair work in their stores.
Accordion Rose Rose Trick Oliver is the daughter of the late Al Trick who manufactured accordions. He was the last manufacturer in the United States to make all his parts, including reeds. Rose started helping him in his shop when she was young. She had a natural talent for tuning reeds, so she did all his tuning. Now she has a repair shop at Port Aransas, Texas, and travels around to accordion festivals where she lectures on accordion repairs.
Ike's Accordion Boxpital A lot of practical information here. Located in Silver Spring, Maryland.
The Ins and Outs of the Accordion This is a great book on accordion repair and maintenance, particularly on reeds. The order goes to a Pacific island and the book is shipped from Paris, France, but it takes only 10 days to two weeks for it to be delivered in the United States.
Sam Shublom For reed wax, reed leathers and other parts for serious work.
Harmonikas.cz A reed manufacturer in the Czech Republic.
Galassi Bellows Bellows manufacturer.
Klingspor's Woodworking Shop Klingspor sells shellac flakes. Drop some of these into denatured alcohol and they will eventually dissolve. When the solution is thick enough it can be used for mounting reed leathers. They have a lot of other goodies, too.
The Japan Woodworker If you want to build your own instrument like, say, a Cajun accordion, this place has some of the finest tools in the world. I have been using Japanese wood chisels for more than 20 years and would not use any others. They also have some nice kitchen knives. I cook two or three days most weeks and have used the same Japanese knife for more than 30 years. I'll be happy to answer any questions about Japanese tools.
Wood Carvers Supply All sorts of small carving tools here, and a large selection of rotary carving tools. I use their stuff to make my musical spoons and other oddities. If you want to tool up to make a violin, this is the place to go.
Wall Lumber This North Carolina company has many species of imported and domestic hardwood lumber, from two inch or more rough cut down to a quarter inch sanded on both sides. Also some nice plywood.
Online Metals Interesting source of small quantities of metal and plastic. If your instrument has some missing buttons, you might find suitable material here to make replacements.