If you mean new, 36" wide by 36" tall by 36" deep and made of stainless then you are looking at around 20 grand of custom build.? Vacuum and large flat surfaces are not compatible so you may want to think about how big a cylinder you need.? Figure another 20 grand for the rough pump, diff pump, backing pump, valves, gates, intercooler and some way to tell what the vacuum level is. ?
Scrounging off Ebay may score you something close but it will never be exactly what you need and you may have to get very resourceful to make it work.?What are you doing with the chamber ??? ? It makes a huge difference as to what it going to cost you to get there.?? ?
I was looking for a 36" dia by 36" deep chamber when I came across my 68" dia x 68" deep chamber on Ebay.? It was old, trashed by a fire, carbon steel, full of scrap metal and dead animal parts and did I mention trashed by a fire ......... but it was 1/20 the price of a new one that big.? Buying used everything off Ebay I still managed to dump the price of a new car into the project to be able to coat a telescope mirror.