British with some imports. Mainly available as white buttons, sometimes as more open ‘cups’, sometimes as fully open ‘flats’. The distinctive brown cap variety has the most flavour.
Frozen usually have an unpleasant spongy texture, although very small ones are sometimes alright sautéed in butter from frozen. Dried sometimes have a surprisingly good flavour in stews and so on. Tinned have a distinctive flavour which you either like or not.