The supplies are amazingly cheap, on the order of a couple of bucks a piece and small, on the order of 1/2 x 3/4 x 1/2. They hook directly to wall power, 85 - 265 VAC is not uncommon. And, they're available all over the place.
One could put an entire arduino, power supply and all in a wall wart size package to do some project or other.
Such an item (cheap, small, common) doesn't exist for constant voltage supplies unless you gut one of those tiny wall warts, and that's a pain sometimes.