Hi Jeff,
It would appear that the Midland and the Bendix units are very similar, both called Hydrovac.
I got my Midland brake booster from BSC Parts Warehouse, 824B Memorial Drive SE, Atlanta, GA 30316 (800) 462-0062.
http://www.brakespecialty.com/brake_boosters Part number is: # HAL-C3400. The list price was $900, net $518.69 plus $21.95 for an installation kit. I think this is the same unit that is used on (at least) 1957 Thunderbirds. There are a variety of this kind of unit available. I did an internet search for “Hydro-Vac Brake Booster” and came up with several hits.
Years ago I installed a Bendix Hydro-Vac booster on my 1947 Continental. I think it was from a Cadillac, but that was in 1969. I probably got it from a junkyard at that time.
Look around – there’s gotta be a cheaper one available somewhere. My local friend got his booster from the same BSC people for his 1948 Continental. His is a different model: SC142C and is slightly larger, but was a bit cheaper.