Wired Lavalier $
A cheap way to get decent audio. Attach the mic to your speaker's lapel and plug it into your iPhone or camera. Away you go! A must have.
Rode Video Mics $$
Pick from any of these on-camera mics. You can also attach these to a tripod to get them closer to your speaker. (But don't forget the extra 1/8" cable extension.)
Zoom H6 External Audio Recorder $$
This unit captures high-quality audio to SD cards. Because you're not tethered to the camera, you can place the unit whereever you need it. Also great for field recordings.
Rode NTG3 Shotgun Mic $$$
One of the best mics you can buy. You'll need an XLR cable and an external audio recorder, or a camera with XLR conncections.