Try eBay too. They usually have a concertina or three up for auction. But the big decision to start with is what system to go for:-

* English (produces the same note on the push and pull motion, is fully chromatic so ideal for song accompaniment but it is not so easy to do chords at the same time as playing the tune and is not so "swingy" for dance music);
* Anglo (produces a different note on the push and pull motion, is more popular for dance music and hence a bit more expensive but, like a melodeon, it is not fully chromatic); and
* Duet (is the best of both worlds but larger to carry, more expensive and some say more difficult to learn).

I suggest that you find a few players near you and have a look see at what they do and how they feel about their instruments. It all depends on what your musical history is (eg if you play a mouth organ or a melodeon now it will be easier to learn the Anglo) and what you intend to do with it ...

Once you have some idea of what you want, you could do worse than contacting Chris Algar, who is based in Stoke-on-Trent in England and is the biggest concertina dealer in the world. I'm sure he would be give you a good deal for a fair price - he sold me mine! Anyway, happy squeezing ...

"Dancing and singing, bell ringing ..."
Alan
PS My pride and joy is a Lachenal Edeophone 56 button extended treble c1902.