Hi Simon,
It is fairly readily available but seems to vary in quality a lot so I tend to buy it in person rather than by mail order. Also as I generally buy it in log form, I try to either weed out any split logs or be selective about which ones I buy so I can maximise my use without wasting any wood.
Unfortunately, masur birch also seems to go up in price every time I look but it's such a nice wood to work with, I just can't resist!