You can try rode smartlavs Theatre about 70$ each. Or do as I did:I bought four Chinese Mics for a buck each on the Internet, you need an adapter to plug them into your iPhone (15). Now the problem with these is: they are decent Mics however without any sort of filter or anything they tend to saturate, so you have to do a few tries on positioning and levels before you start shooting. Once you start shooting you'll have no way of monitoring the sound so after the take before you send the actors home just listen to the takes. You know what they say you can have cheap, fast or good, pick two, this is a little slower but it's good and cheap.