I've been trying out some cheap, entry-level mics for mojos on a budget. Here is a low-cost shotgun mic called the Takstar SGC 598. It's not up there with the more expensive models, but it's not terrible.
The Takstar SGC 598 is a cheap shotgun mic. With a TRS to TRRS adapter it can be made to work with a smartphone. It comes with a battery and requires a cold shoe mount to mount it on your phone.