It depends how much can you things yourself.
You may do it in this way:
1. Buy a unit
2. Check the size of you window and order specialists in windows to produce the frame which would suit the air conditioning unit.
3. You should weld or make in other way a supporting contruction which will be outside of your window. The conditioner will be laid on it by one side. Thats it !
I had wooden frame so I found a frame in a store similar to mine. I cut the frame and made a necessary shape using parts of the new one. I also cut the glass to fit a new size. In general it is better to choose a window with size close to the size of a unit. you need less efforts to resize the frame. Good luck