Alder is readily available, at least in smaller sizes. Check wood turning suppliers. Not sure if I'm allowed to post retailers or not bit Okenden Timber at Church Stoke sell it they have a website also.
A few years ago there was a scare about a disease that would wipe Alders out - but I never heard of any such thing actually happening and I still see them about so there should be no shortage.
It's weird stuff in that its wood provides very little heat as firewood and yet its charcoal burns very hot. As such it is in great demand from charcoal burners. Maybe they snaffle it all before timber merchants get a look in?
I'll ask at Mac Timbers next time I'm there.