Depends on what you are needing to use it for. If you just want to set channel levels, then you can just use an app on your phone. Literally.
The exact SPL level doesn't need to be accurate for doing this, it just needs to be consistent from 1 speaker to the next. So the mic on your phone with an app, works perfectly fine for this.

Now if you are wanting to set the subwoofer channel more accurately (which requires much better low frequency response than a phone microphone can do) or set the system to reproduce sound at a calibrated volume (either 75dBm or 85dBm). Then you need dedicated calibrated microphones.

For cheap, I like using the iMM-6 mic with the AudioTool app:
[Link: play.google.com]

For more serious, I recommend this:
https://www.studiosixdigital.com/

And the gold standard (if for no other reason than price & complexity), Goldline TEF25:
[Link: gold-line.com]

For old school: I to have some of those old dedicated SPL meters laying around also. I also have a Phonic PAA3 collecting dust.