I was talking with a heat-pump and air-conditioning contractor near Monteal a few weeks ago and asked about the outdoor "condenser units" that are now typical for home owners. They have a sealed motor/fan component that forces air through the heat exchanger within the unit. Apparently, when these condenser units fail the whole unit is replaced with a new one and the unit is usually scraped even though it is most likely that the motor/fan component is completely okay. This installer indicated that hundreds of units could be salvaged in good working condition from just the normal service that they do each year.
This looks like a good source at low cost of BG Fans and I recommend looking in your own neighborhood for a source of salvaged equipment of this type. They may be a bit underpowered and I highly recommend that the Bubble Generator System? is configured with two units at each end of a Tunnel Greenhouse, with placement within the Cavity Space at the ends of the Roof Truss? (structural component) which usually would extend over or along side the Door Way? at the End Wall?. This creates an easy push and pull of bubbles in a straight Bubble Flow Path that produces very little "back pressure" and the tube axial fan type can do the job with a fractional HP motor (say, half a Kw).
The above is a good solution for an AC fan unit. For a DC electric power BG Fan? we can use the automotive radiator motor/fan units. Again, sealed motor units are best and necessary for placement of the Bubble Generator? within the Bubble roof or wall, which is a wet environment.