Make a pound cast if you want to "see" just the throat. Bonus: You can also measure what you see.
To answer your question, I got the 20-caliber one for Android and PC for $49. Het creative with eccentric bushings for calibers over .30. Only one kind of software worked for me on my phone, it's called "endoscope camera", got it from g**gle play. OTG was rated much higher but did not work. You have to give it all the access permissions it asks for or it won't work.