BMX, kids bikes, and cruisers, use them. Any bike shop can get you one, if they really want to bother. I use a smallish BMX coaster brake hub (no brand name on it) laced to a decent 700c wheel. It's on a bike with no front brake. It works fine for me, however I take it easy in most places with that bike. It does not stop very well, I anticipate way ahead and slow early, similar to a fixie. I have no idea what would happen if I really started hammering on the thing.
There's no way I could stop as fast as a bike with two hand brakes, but it is a blast to ride this bike, I love it. I don't try big hills with it either.

Shimano still makes them. Retailers typically only sell things that are in demand. If they don't sell well, some people won't offer them for sale. But there's plenty of them around.