Most developers realize that having a high fidelity mockups are really useful to visualize the final product.
However creating high fidelity mockups for developers is an additional skill that not all have.
I have found few options to come up with high fidelity mockups for mobile developers (for both iOS and Android developers) and have created a free course on it.
Hit up this link and sign up for the free class to learn more.
This course also teaches how to create mockups for desktop and web projects as well.