We use chambers from all of the already mentioned vendors, along with Thermomtron
http://www.thermotron.com
Also, check out Scientific Climate Systems
http://www.dryrooms.com/environ/index.html
The combination of -40, flexible, fire resistant, and 150,000 btu/hour heat load is going to be a tough combination. There are not many materials that stay flexible at that temp. The heat load is going to require multi-stage cascade cooling, or liquid nitrogen (or CO2) assist. It also will need to be purged out with some sort of dry gas, or everything inside will suffer the "popsicle effect".
Does the entire chamber need to be flexible, or just one side? We have several chambers that are set up with rubber floors to allow vibration testing to be performed. The other 5 surfaces are stainless steel and insulated.
-jeff