Welcome and good luck with your moto mod development. The Raspberry PI Hats have been out of stock for a while now and I am not sure if there are any more being made. You might have to scour the internet to find them as they were very easy to start a prototype. If you have time and opportunity the moto mod example attachments are a great way to start and get some familiarity and the blinky example might be helpful as well.
There are a few people with experience building prototype mods but not as many as before so it might take a while to get some help or assistance but I think learning about them is a good way to get experience.