how to find frequency response of microphone
This is unfortunately a difficult problem.

A microphone doesn't have a single frequency response. It has a different one for each direction of incidence. Especially for second order microphones (dipoles, cardioids, etc.), the frequency response also depends heavily on the distance and radiation impedance of the source. So first you need to decide: what distance and do you want on-axis, frontal averaged or diffuse field response
Measuring the microphone directly requires an EXTREMELY well behaved loudspeaker. Most microphones are very linear and quite flat, but most speakers are definitely not. A measurement grade speaker is hard to come by and very expensive.
If you really just want the microphone response you will also need an anechoic chamber that covers your frequency range of interest. Otherwise the response of your acoustic environment (room, walls, furniture, etc.) will dominates the microphone response. Buying a a few of those will seriously strain your relationship with the CFO of even a mid-sized high tech company. Don't ask me how I know :-)

Your best shot is to find a good reference microphone that has the same directionality type of the microphone you want to measure. Than excite a loudspeaker with pink noise and measure the power spectrum for both the reference mic and the your test mic at EXACT the same location and orientation. Average over many positions, rooms and a few different speakers.
The frequency response of your microphone is the difference of the mic's power spectrum to that of the reference microphone.