Datsv3 vs Umic1

Long story short. I have a DatsV3 gifted to me by one of my automotive students.
Im now in need of a calibrated mic.
Is there any guides as to how I can buy a cheap mic and create my own calibration file. What if I buy just a Mic pickup and then what kind of circuit would I need to build around it.
Next question it would seem that the Dats unit has a sound card. Has both an amplifier and a mic. i.e. both input and output. Capabilities.
In fact I have setup True RTA and am able to use use my Dats audio input (It shows up as a USB Audio Codec).
Also read the article on how to use the dats with REW on a mac. Plan to see if I can use the same steps on windows.

Has anybody done this on windows ?.
Whats the most economical way to build a mic that I can use with REW or TrueRTA. Or any of the other spectrum analyser apps.
Im in the process of trying to see how accurate these spectrum analysers are on my phone when I get my hands on a friends Radio Shack DB meter. Most have a peak hold function. And some even allow you to save the results to a txt file. On the phone.
Some of the android apps I have tried are
Spectrum Analyser Pro / Spectroid / SpectralPro / RTAPro /
Im trying to see if I can sort out my tool box without having to invest in a 100-200$ Mic.
Not sure why but info on Dats seems to be very sparse.