2 options....
https://www.amazon.co.uk/Professional-Omnidirectional-Recording-Condenser-Microphone/dp/B07XRS1RDC/ref=sr_1_1?dchild=1&keywords=lavalier+for+iphone&qid=1600671325&sr=8-1
there are many available, and you may be able to get one that has 'gain' control (increase sensitivity), but chances are any of them will improve upon the built-in mic.