Tiny Cam Pro is still available on the Amazon App store:

https://www.amazon.com/tinyCam-PRO-Swiss-knife-monitor/dp/B004VD3YIM

However, if it won't work on your firestick, it won't show up in results when you search for it.

I suggest you try my link from a computer and look on the right side which of your devices it is compatible with.

I've got the app and it is only compatible with some of my firesticks and I don't know why.

I've got a few 4k sticks and it says it isn't compatible with all of them. I think they are all the same generation but maybe not. I've not gone to each to see what fireOS they are all on to see if they are the same.