Erica arborea is one plant known as tree heath, I believe it is what I saw as trees in Ethiopia, growing among the tree-sized St John's wort, in the Simien mountains. Erica lusitanica is also sometimes called tree heath here too.

I was under the impression that E arborea was widely available. I've certainly purchased plants of that label in packs of 6 random heathers from ordinary places without really trying. But they've never grown to any size for me, they remain low little shrubs, utterly unlike the trees I saw in Ethiopia. I also have an E lusitanica, which has periodically lost branches in relation to building works, which has knocked it back a bit. But given the rate at which it grows, I don't think it was ever going to get very big for me.