Old furnaces nearly always have big-ish squirrel cage blowers in them, and many are replaced before the fan motors fail for various reasons. In my area they ar

Old furnaces nearly always have big-ish squirrel cage blowers in them, and many are replaced before the fan motors fail for various reasons. In my area they are a reasonably frequent sight by the side of the road on bulk garbage day. If you get there before the scrap metal guys do, you could easily nab a blower. They're typically 120V single-speed devices, although some newer ones may have variable-speed fans. Not 100% sure how most of them vary the speed; my bet is that they just have some capacitors switched inline with the fan motor to add reactance to the circuit and slow it down. (That would probably be how I would slow down a single-speed one to make it quieter, at any rate.) There are a lot of projects online that you can find, showing how to build fairly big workshop air cleaners with a furnace blower and some cheap plywood/MDF/whatever. Basically you just build a 5-sided box around the blower with a hole for the outgoing air, and then mount a furnace filter on the open face of the box.