Mrs. WoT has had quite a bit of luck at car boot sales - nice nearly-new Hohners, etc.
Take a chromatic guitar tuner so you can check it's in pitch (and relative pitch). Hold one half of it and let the other half drop to the floor, to check the bellows are air-tight. The slower the other half drops, the better. If there's a leak, it'll fall more quickly.
As mentioned above, don't buy anything exotic that looks like it should be nailed to a pub wall.