Looking at the sear ( part 41) if you have a dremel or if not swiss files, you can elongate the oval hole ( elongate it to the left as you look at the diagram) this will give you more sear engagement. Only elongate by about 1mm. I have done this myself to one of these pistols in the past with excellent results.
Otherwise you can try Manchester Airguns ,who will order the part for you or John Knibbs also.
pete