Option #1: Clip-on Wired Type. Audio-Technica ATR-3350IS Omnidirectional Condenser Lavalier Mic with Smartphone Adapter. https://amzn.to/2Nrrht9 (US$29)
I choose this because I know Audio Technica for like 20 years in the video production industry. I have four of these long-wired clip-on mics.
Option #2: Clip-on Wired Type. IK Multimedia iRig Mic Lav compact lavalier microphone. https://amzn.to/2u8JGSA (US$50)
Great for interviews and other two-source situations, iRig Mic Lav offers a chainable design. Just connect one microphone to the second and you're up and running instantly. No mixer or adaptors required.