That's how I've seen it done. A small shop-vac blowing into the bottom of a brake drum. They put a thing with a hole in it below the brake drum. A flap over the hole can be pushed to the side to let some of the extra air out, to adjust the amount of air that actually gets blown into the coals. I've heard of people using old furnace blowers too.

I've seen some older ones with a hand-crank blower, too, but I don't know if they make those anymore. Well, I imagine the Amish probably still make them, but they'd be expensive for the non-Amish to buy!