First, to a doctor.
To a doctor in the CHI system to clarify whether this operation is really necessary and whether it can be carried out under the CHI policy.
With a doctor, you can discuss where exactly you can receive paid assistance (if free assistance is not possible), and then clarify with your insurance company how to reimburse the cost of treatment. Let me remind you that within the framework of the CHI you have the right to a fairly wide range of assistance, which includes, among other things, surgical treatment, including quite complex diseases.
And now, if you were finally denied the CHI system , and you are sure that the operation is necessary, contact the specialized fund. Almost any sane fund will ask you if you tried to solve your problem within the framework of the CHI.
The exact recipient of the request depends on what kind of operation is required. In principle, there are several databases, but all of them are incomplete.
The RusFond Navigator is probably the most complete, it also makes sense to contact All Together
In general, the algorithm for contacting a charitable foundation is described here.