[Blackbrook Eye]

My suggestion, taking into consideration your specific needs Dai would be a Rowan it is relatively fast growing and hardy and produces berries that last for a good length of time, assuming the birds don't eat them all at once.
I have had one growing in my garden for more tha fifteen years and although initially it grew quite rapidly, it has not grown out of proportion and does not look likely to, all of my feeders are attached to the lower branches for which it is ideal and I get a pretty good variety of birds visiting, including during the recent cold spell a few Redwings.
As for where to acquire a sapling, I suggest that you contact your locol branch of the Woodland trust for local information, I have found them to be most helpful.
Hope this is of use to you.

Keith