Mostly you make such mirrors by yourself or with the local astronomy's club mirror making group/guru if there is one.
There are some nice advantages with long focal length scopes and they are really worth building.
Their problem is that the long length also means you get very familiar with large ladders and how to climb them and move them about in the dark.
I'll note that there are a fair number of webpages devoted to mirrormaking craft and perusing a number of them will tend to show how easy it really is to make a mirror.