I wouldn't buy any used bunny boots. Bunny boots are called Vapor Barrier boots by the military. The insulation between the inner and outer layer or rubber is in an airtight chamber. If there is a pinhole in the either the inner or outer layer of rubber moisture can get inside and soak the insulation. You won't know if that happens until you get frostbite. When I was in stationed at Ft Rich in the 70's we sent our boots in to be tested every fall. At least one boot failed every year and came back with a red stamp on the toe. The military replaced those boots and sold the bad boots as surplus. Most of the used boots at the surplus stores have that red stamp. I don't think the military uses bunny boots any more. You can buy new bunny boots for a couple hundred bucks but again your not going to know if you get one of those pinholes that will compromise the insulation. The main advantage of bunny boots is that they will keep your feet warm even if they are soaking wet, like if you step through the ice. For flying I think there is less risk of stepping through the ice but I guess it could happen. There are other boots out there that will work as well or better than bunny boots.
Take a look at the Wiggys products. The web site is wiggys.com. There is a Wiggys store on Old Seward Highway near Diamond in Anchorage.
You also might want to think about a fire resistant layer next to your skin. Helly Hanson makes insulated long underware that will not support combustion and will turn to ash rather than melt when exposed to high heat. It's made of something other than nomex. Nomex is great stuff but it will not keep you warm. The problem with the synthetic clothing is that it burns like oil (because it is oil) and melts when exposed to high heat. If you have ever seen the result on a person you will never forget it. I bought several pair of the Helly Hanson underwear at Eagle Enterprises on International Airport Road in Anchorage.