Here are some websites that I have found useful:

concertina.net - this is the best and most complete website for anything concertina related.

concertina.com - a very good collection of links to various concertina related websites

concertina FAQ - a comprehensive collection of questions and anwers related to the concertina.

The Button Box - a dealer in new and used concertinas. Also performs repairs, tuning and restoration work.